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BROOKLYN,  N.  Y.,  SEWAGE 
TREATMENT  EXPERIMENTS 

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By 

GEORGE  T.  HAMMOND,  M.  Am.  Soc.  C.  E. 

156  BERKELEY  PLACE,  BROOKLYN,  N.  Y. 


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REPRINTED  FROM  THE 

PROCEEDINGS  OF  THE  AMERICAN  SOCIETY  FOR  MUNICIPAL  IMPROVEMENTS 

NINETEEN  HUNDRED  AND  NINETEEN 


BROOKLYN,  N.  Y.,  SEWAGE 
TREATMENT  EXPERIMENTS 

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A  Brief  Review  of  Five  Year  s  Work 

By 
GEORGE  T.  HAMMOND,  M.  Am.  Soc.  C.  E. 

156  BERKELEY  PLACE,  BROOKLYN,  N.  Y. 

Engineer   in   Charge    of   Experiments 
and  of  Sewer  Design,  Brooklyn,  N.  Y. 

Surgeon  (Reserve)  U.S.  Public 
Health  Service 


REPRINTED  FROM  THE 

PROCEEDINGS  OF  THE  AMERICAN  SOCIETY  FOR  MUNICIPAL  IMPROVEMENTS 

NINETEEN  HUNDRED  AND  NINETEEN 


BROOKLYN,  N.  Y.  SEWAGE  TREATMENT 
EXPERIMENTS 

A  Brief  Review  of  Five  Years'  Work. 

By  George  T.  Hammond,  Surgeon  (Reserve),  U.  S.  Public  Health 
Service,    Consulting    Engineer,    156    Berkeley   Place,    Brooklyn, 

N.  Y.  

Introduction. 

Bearing  in  mind  the  brevity  required  in  a  paper  intended  for 
presentation  at  a  Convention  of  this  Society,  it  seems  proper  to 
begin  it  with  a  statement  concerning  the  subject  matter  and  the 
limits  within  which  it  is  confined. 

The  Boro  of  Brooklyn  has  been  making  sewage  treatment 
experiments  for  a  number  of  years  past.  These  experiments 
were  interrupted  and  brought  to  a  close  by  the  war,  at  the  begin- 
ning of  1918,  when  the  writer  left  the  city's  service  to  enter  the 
war  service  of  the  U.  S.  Government,  as  sanitary  engineer  to  the 
U.  S.  Shipping  Board,  Emergency  Fleet  Corporation,  and  later 
to  the  U.  S.  Public  Health  Service. 

Before  leaving  his  duties  with  the  city,  the  writer  made  a  com- 
pilation of  the  data  obtained  from  the  work  of  the  sewage  treat- 
ment experiment  station,  at  the  request  of  Mr.  E.  J.  Fort,  chief 
engineer  of  sewers,  who,  as  one  of  his  last  official  duties  before 
leaving  the  city's  service  to  become  city  manager  of  Niagara 
Falls,  prepared  during  the  summer  of  1918,  a  final  report  on  the 
26th  Ward  Sewage  disposal,  for  presentation  to  the  boro  officials. 
The  writer  is  informed  that  this  report  is  being  prepared  for 
the  printer  and  will  soon  be  available  to  those  who  are  interested 
in  the  details  of  the  work,  the  conclusions  reached,  and  the  recom- 
mendations made  for  the  solution  of  the  problems  for  which 
the  work  was  undertaken. 

Soon  after  the  work  of  the  Experiment  Station  was  initiated, 
Mr.  Fort  informed  the  writer  that  he  had  promised  to  have  pre- 
sented to  this  society  such  information  in  the  form  of  papers 
as  might  be  available  at  the  time  of  the  annual  conventions.  It 
was  in  accordance  with  his  instructions  that  the  following  papers 
were  prepared  and  presented  by  the  writer : 

Sezvage  Treatment  Experimental  Plant  in  Brooklyn,  N.  Y. 
Presented  in  1914  at  the  Boston  Convention. 

Sewage  Treatment  bx  Aeration  and  Activation.  Presented  in 
1916  at  the  Newark  Convention. 

To  complete  the  series  of  papers  on  this  work,  the  present 
paper  was  planned  in  1917,  and  was  to  have  been  presented  in 

3 


4  Brooklyn,  N.  Y. 

1918;  but  on  account  of  the  war  this  intention  could  not  be  real- 
ized. It  has  recently  been  revised  and  made  considerably  more 
brief  than  it  was  as  completed  in  the  spring  of  1918;  and  yet 
apology  seems  due  for  its  considerable  length. 

The  purpose  of  the  writer  has  been  to  treat  the  subject  wholly 
as  a  study  in  sewage  disposal,  far  removed  from  any  official 
character.  If  it  were  possible  to  eliminate  from  it  all  reference 
to  the  special  problems  for  which  the  plant  was  provided,  this 
would  be  done,  as  the  important  matter  for  us  in  such  a  study 
is  to  look  for  the  underlying  principles  of  general  application,  if 
any  such  there  may  be.  If  such  principles  are  not  present,  the 
study  can  be  of  but  little  if  any  interest.  But  concrete  facts'  are 
necessary  for  illustrative  data.  We  should,  however,  beware  of 
data  so  local  and  dependent  upon  the  concrete  conditions  afforded 
by  local  problems  that  they  would  not  apply  elsewhere.  The 
final  report  on  this  work,  when  published,  will  go  over  the  entire 
ground  from  the  standpoint  of  the  local  problems  and  with  all 
the  difficulties  involved  in  meeting  local  conditions.  It  will  con- 
tain much  of  no  interest  to  the  engineering  profession,  along 
with  many  facts  and  data  out  of  which  interesting  general 
principles  can  be  laboriously  extracted.  With  its  local  conclu- 
sions and  recommendations,  this  paper  will  have  nothing  to  do. 
But  the  attempt  will  be  made  herein  to  present  some  of  the  facts, 
selected  carefully  from  that  voluminous  mass  of  material,  which 
the  writer  hopes  and  believes  will  prove  of  interest  as  a  basis 
for  general  discussion,  thereby  carrying  out  the  original  idea 
of  Mr.  Fort,  as  explained,  when  he  asked  the  writer  to  prepare 
the  series  of  papers  of  which  this  is  the  final  one. 

When  the  experiment  station  was  authorized  and  constructed, 
and  also  thru  its  period  of  operation,  Hon.  Lewis  H.  Pounds 
was  President  of  the  Boro  of  Brooklyn,  going  out  of  office 
January  1,  1918,  and  Mr.  E.  J.  Fort  was  Chief  Engineer,  from 
which  office  he  resigned  to  become  City  Manager  of  Niagara 
Falls,  N.  Y.,  late  in  1918.  The  writer  was  engineer  of  design 
of  the  Bureau  of  Sewers  and  in  charge  of  the  experiment  station 
under  Mr.  Fort.  Mr.  W.  T.  Carpenter  was  chief  chemist  of  the 
plant  1913-1917,  and  Mr.  Murray  P.  Horowitz,  assistant  chemist. 
Mr.  George  H.  Knight,  assistant  engineer,  assisted  the  writer 
in  all  engineering  matters,  and  for  a  considerable  time  repre- 
sented him  at  the  plant. 

Dr.  Rudolph  Hering  gave  expert  advice  and  revised  the 
plans.  Dr.  Karl  Imhofr  revised  the  plans  of  the  Imhoff  tanks 
and  gave  much  other  advice.  Professor  Earle  B.  Phelps  gave 
expert  advice  on  sewage  aeration.  Much  assistance  was  given 
during  the  work  by  many  visiting  experts. 

The  points  that  will  be  briefly  touched  on  are : 
(1)   Some  general  remarks  about  the  plant  and  the  local  con- 
ditions. 


Sewage  Treatment  Experiments  5 

(2)  A  statement  concerning  the  laboratory  technic  and  sampling. 

(3)  The  methods  of  measurement  employed. 

(4)  The  sewage,  how  obtained   for  the  experiments,   its  quan- 

tity per  capita,  and  most  important  characteristics. 

(5)  Some  remarks  on  the  operation  of  Imhoff  tanks,  trickling 

filters,  and  settling  tanks,  sludge  digestion  and  sludge 
drying,  giving  results  obtained  in  very  short  retention 
periods,  and  by  very  rapid  operation  of  the  filter  beds. 

(6)  Some    studies   on   the   operation    of    Riensch-Wurl    screens 

and  the  application  of  screened  sewage  to  trickling  filters. 

A  Brief  Review  of  the  Work 

Sewage  investigations  were  commenced  at  this  station  in  1910 
by  Professor  Earle  B.  Phelps,  and  General  (then  Colonel)  Wil- 
liam M.  Black,  Corps  of  Engineers  U.  S.  A.  Their  report  on 
the  aeration  treatment  of  sewage,  now  a  valued  classic  in  sanita- 
tion literature,  was  presented  in  1911.  The  studies  were  con- 
tinued by  the  Bureau  of  Sewers  under  Mr.  E.  J.  Fort,  assisted 
by  the  writer,  and  in  1912  an  extensive  experimental  plant  was 
authorized.  At  the  same  time  a  new  sewage  disposal  plant  was 
provided  for  by  the  Board  of  Estimate — the  design  to  await  the 
results  of  experimental  study.  Local  conditions  were  given  care- 
ful study,  and  all  American  sewage  disposal  plants  of  note  were 
visited  by  Mr.  Fort  and  the  writer. 

During  the  spring  and  summer  of  1912,  Mr.  Fort  and  the 
writer,  in  company  with  Dr.  Rudolph  Hering,  visited  the  more 
important  sewage  disposal  plants  in  Great  Britain  and  on  the 
European  continent, — taking  in  all  of  the  principal  cities  in  Eng- 
land, Germany,  France,  Switzerland,  etc. — gathering  data  for 
the  design  of  the  experimental  units  and  also  for  the  permanent 
plant. 

In  1913  the  experimental  units  and  laboratory  were  completed, 
except  the  screens,  which  were  not  placed  until  1916.  Operation 
of  the  three  Imhoff  tanks,  plans  of  which  had  been  revised  and 
approved  by  Dr.  Karl  Imhoff  himself,  was  started  in  1913,  and 
at  the  same  time  all  of  the  trickling  filters  and  aeration  experi- 
ments. This  work  was  continued  through  1914  and  1915,  and 
indeed  until  the  close  down  of  the  plant  in  January,  1918. 

Sewage  aeration  experiments  were  undertaken  concerning  ac- 
tivated sludge  from  1915  to  1918. 

The  Riensch-Wurl  screens  were  installed  in  1916,  and  experi- 
ments with  them  continued  to  1918. 

There  was  much  other  experimental  work  done  on  various 
other  methods  of  sewage  treatment,  and  on  variations  proposed  in 
the  standard  methods,  but  this  work  may  be  considered  as  rela- 
tively unimportant,  and  led  to  no  conclusions  worth  stating  here. 


Brooklyn,  N.  Y. 


Sewage  Treatment  Experiments  7 

It  may  be  observed  that  every  kind  of  an  experimental  unit 
designed  was  itself  an  experiment.  It  was  only  by  installing  such 
a  plant  that  the  actual  conditions  under  which  it  would  be  oper- 
ated could  be  fully  discovered.  It  was  anticipated  by  the  writer 
that  as  much  could  be  learned  from  the  troubles  and  limitations 
discovered  in  operation,  as  from  the  successful  performance  of 
the  different  units.  Long  and  careful  study,  and  many  changes 
in  experimental  units,  are  required  in  order  to  obtain  dependable 
results  in  these  studies,  and  a  careful  recognition  of  existing  con- 
ditions and  circumstances  under  which  a  plant  shall  operate,  and 
the  adaptation  of  the  plant  to  these  conditions,  is  essential  to  the 
success  of  these  investigations. 

The  experiment  station  consisted  of  a  laboratory  for  the  chemi- 
cal and  bacteriological  work  connected  with  the  study  of  sewage, 
and  a  sewage  treatment  experimental  plant.  The  size  of  each 
of  the  various  units  of  the  plant  was  made  large  enough  to  permit 
operation  on  a  scale  sufficiently  great  to  take  it  out  of  the  class  of 
a  mere  laboratory  test.  All  of  the  work  was  carried  out  on  such 
a  scale  that  each  experiment  was  made  under  conditions  designed 
to  approach  actual  sewage-disposal-plant  conditions. 

The  plant  included  three  Imhoff  tanks  of  different  depths,  but 
with  other  corresponding  dimensions  equal ;  six  trickling  filter 
beds,  two  of  which  were  designed  for  discharging  compressed  air 
within  the  mass  of  the  stone  medium ;  various  tanks  and  appara- 
tus for  the  investigation  of  sewage  treatment  by  forced  aeration ; 
secondary  settling  tanks  for  the  trickling  filters,  and  for  the 
sewage  treated  by  aeration ;  a  plain  settling  tank  for  crude  sewage 
in  connection  with  an  airtight  sludge  digestion  tank,  which  re- 
ceived the  settled  matters  and  sludge  from  the  settling  tank,  being 


Fig.  2.     View  of  Plant  When  Put  in  Service,  Oct.  1,  1913. 


8  Brooklyn,  N.  Y. 

in  effect  the  two  essential  portions  of  an  Imhoff  tank  separated ; 
ten  sludge-drying  beds,  and  various  units  for  experimenting  with 
fine-screened  sewage,  and  for  drying  sludge. 

The  mechanical  plant  consisted  of  steam-actuated  sewage 
pumps  and  an  air  compressor ;  with  attachments  and  appliances 
for  measuring  volumes  of  air  delivered. 

The  pumps  that  furnished  the  sewage  for  the  experiments  were 
located  within  the  existing  sewage  disposal  works  buildings,  and 
steam  was  obtained  from  the  boilers  of  that  plant.  There  were 
two  sewage  pumps,  both  of  direct-acting  piston  type,  so  installed 
that  either  one  could  be  cut  out  and  cleaned  or  repaired  without 
stopping  the  other.  The  larger  pump  had  a  capacity  of  1,200,000 
gallons  per  day,  and  the  smaller  650,000  gallons. 

The  compressor  installed  was  a  duplex  crank  and  fly  wheel 
machine ;  automatic  in  starting,  stopping  and  speed ;  with  a  dis- 
placement capacity  of  228  cubic  feet  of  air  per  minute  at  not  ex- 
ceeding 210  r.p.m.  for  30  lb.  air  pressure  and  with  100  lbs.  of 
steam  at  the  throttle.  It  was  equipped  with  a  combination  speed 
and  pressure  governor,  arranged  to  bring  the  machine  to  a  dead 
stop  if  no  air  was  demanded,  and  to  start  up  automatically  upon 
drop  of  the  pressure. 

The  first  thing  determined,  in  making  the  design,  was  the  re- 
quired elevation  of  the  surfaces  of  the  sewage  in  each  unit  of 
the  plant.  This  determined,  the  unit  was  designed  to  comply 
with  it. 

The  low  elevation  of  the  ground  at  the  site,  but  a  few  inches 
above  ordinary  high  tide— a  tidal  marsh,  in  fact — necessitated 
the  placing  of  the  experimental  units  above  the  reach  of  the 
highest  tide,  and  pumping  the  sewage  up  to  such  a  level  that  a 
gravity  flow  could  be  obtained  for  every  unit  of  the  plant. 

The  datum  line  was  at  mean  high  water  and  the  surface  of 
the  sludge  beds  was  placed  at  an  elevation  of  2.67  feet  above  this 
datum.  All  of  the  other  units  of  the  plant  were  given  such  eleva- 
tions above  this  as  their  operation  required.  Every  unit  was  pro- 
vided with  a  measuring  device,  for  the  determination  of  the  flow, 
usually  consisting  of  an  adjustable,  calibrated  orifice  above  which 
a  constant  head  was  maintained  by  a  system  of  overflow  weirs. 
Surplus  flow  wasted  back  to  the  main  sewer.  There  was  always 
a  considerable  surplus  flow,  so  as  to  keep  a  good  velocity  in  pas- 
sages. 

For  measuring  compressed  air,  Venturi  meters  were  provided. 
The  accuracy  of  all  the  measuring  devices  was  carefully  tested  in 
place. 

The  construction  of  the  plant,  for  the  most  part  above  the 
surface  of  the  ground,  afforded  opportunity  for  studying  the 
flow  of  sludges  of  different  kinds,  Imhoff  especially,  which  could 
not  as  easily  have  been  studied  otherwise ;  also,  the  effect  of  cold 


Sewage  Treatment  Experiments  9 

weather  on  exposed  sprinkling  filter  beds ;  the  danger  of  freezing 
of  the  various  channels  carrying  sewage,  and  the  proper  method 
of  protecting  and  operating  the  same.  These  studies  were  of 
much  interest  from  the  engineer's  standpoint,  in  view  of  the  pro- 
jected construction  of  a  trickling  filter  plant  on  concrete  pile 
foundations  over  an  extensive  marshland. 

ADJUSTABLE     ORIFICE 


Fig.  3. 


Laboratory  Control  and  Sampling 

The  laboratory  was  erected  as  part  of  the  experiment  station, 
and  supplied  with  heat  and  power  from  the  plant  of  the  existing 
sewage  treatment  works.  It  was  provided  with  the  usual  ap- 
paratus for  sewage  investigations  and  for  the  filing  and  classify- 
ing of  results.  The  laboratory  work  conformed  to  standard 
practice.  The  standard  methods  of  the  American  Public  Health 
Association  were  followed.  The  analytical  work  and  record  keep- 
ing were  organized  by  Mr.  W.  T.  Carpenter,  Chief  Chemist,  and 
the  methods  introduced  by  him  were  followed  thruout  the  entire 
period. 


10  Brooklyn,  X.  Y. 

Samples  having  to  do  with  the  "oxygen  balance"  of  the  sewage, 
and  various  effluents,  are  from  their  nature,  incapable  of  being 
composited.  Effort  was  therefore  made  to  take  them  at  such  a 
time  of  day  that  the  sewage  would  be  most  likely  to  be  of  average 
quality.  The  monthly  averages  from  which  conclusions  were 
drawn  were  averages  of  ten  or  more  samples. 

Samples  for  the  determination  of  such  constituents  as  are  prop- 
erly made  on  composites  were  collected  as  follows : 

A  full  set  of  half-pint  samples  was  taken  every  four  hours, 
midnight  to  midnight,  and  put  into  bottles  with  chloroform. 
These  bottles,  being  the  primary  composites,  were  renewed  daily, 
and  a  pint  portion  taken  to  form  the  secondary  composites  for 
analysis.  The  analyses  of  the  composites  were  never  made  less 
frequently  than  five  times  monthly,  and  ten  times  for  all  the 
more  variable  conditions  of  sewage  flow.  Monthly  averages 
were  determined  by  the  composition  of  about  250  individual  por- 
tions. 

In  order  that  there  may  be  no  misunderstanding  as  to  the 
meaning  of  certain  terms  used  in  this  paper,  which  are  not  always 
given  the  same  definition,  the  following  explanation,  prepared  by 
Mr.  Carpenter,  is  appended : 

Settling  Matter  is  the  volume  of  matter,  solid  and  water  con- 
tent, which  will  settle  to  the  bottom  of  an  Imhoff  settling  cone, 
and  is  expressed  in  cubic  centimeters  per  liter. 

The  time  period  used  in  all  of  these  determinations  was  two 
hours.  After  an  hour's  settling  the  cones  were  rolled  between 
the  hands  a  few  times,  to  send  to  the  bottom  such  of  the  solids  as 
adhered  to  the  sloping  sides. 

By  Total  Suspended  Solids  is  meant  the  gravimetric  amount 
of  dry  material  which  will  be  retained  upon  the  asbestos  mat  in  a 
Gooch  crucible,  after  filtration  with  suction.  Its  weight  is  ex- 
pressed in  parts  per  million  parts  of  the  sewage  from  which  it  is 
taken. 

By  Volatile  Suspended  Solids  is  meant  that  portion  of  the  dry 
material  in  the  Total  Suspended  Solids  which  is  lost  by  incin- 
eration at  cherry  red  heat,  in  a  muffle  furnace.  It  is  expressed 
in  weight  in  parts  per  million,  and  by  per  cent,  of  the  Total  Sus- 
pended Solids. 

It  is  doubtless  more  than  the  organic  matter,  but  the  difference 
is  not  readily  determinable,  and  probably  constant  within  very 
narrow  limits,  and  unimportant,  as  the  figures  are  only  of  inter- 
est in  making  comparisons.  During  the  first  three  years  the  char- 
acter of  the  suspended  solids  was  analyzed  closely  to  determine 
what  portion  of  the  solids  was  settleable,  and  what  portion  non- 
settleable,  or  colloidal.  The  Gooch-crucible  method  was  used. 
The  total  suspended  solids  were  determined  on  an  unsettled  sam- 
ple, and  the  colloidal  solids  on  a  sample  siphoned  from  the  upper 


Sewage  Treatment  Experiments  11 

portion  of  the  contents  of  an  Imhoff  cone  after  two  hours' 
sedimentation.  The  latter  are  called  Colloidal  Suspended  Solids, 
though  they  undoubtedly  contain  a  minute  and  scarcely  apprecia- 
ble amount  of  material  capable  of  settling  in  longer  periods  of 
time.  The  Settling  Suspended  Solids  are  the  difference  between 
the  two. 

Oxygen  Consumed  was  determined  in  accordance  with  the 
1913  Standard  Methods  of  the  American  Public  Health  Associa- 
tion, using  a  30-minute  digestion  period,  the  heating  being  done 
in  an  Arnold  steam  sterilizer.  The  determinations  were  made 
on  raw  and  on  paper-filtered  samples  of  sewage. 

Nitrogen  as  Free  Ammonia  was  obtained  by  direct  nessleriza- 
tion. 

Organic  Nitrogen  was  determined  colormetrically  by  the 
Kjeldahl  process  after  direct  nesslerization. 

Nitrogen  as  Nitrites  was  determined  according  to  "Standard 
Methods". 

Nitrogen  as  Nitrates  was  determined  by  the  phenol-sulphonic 
method. 

This  procedure  was  adopted  after  experimenting  upon  the 
brucine  and  the  narcotine  methods.  The  reduction  method  was 
considered  too  time-consuming  to  be  justified  with  the  force  avail- 
able. Care  was  used  at  all  times  to  insure  the  evaporation  of  the 
last  drops  of  the  sample  in  the  air,  instead  of  on  the  steam 
bath,  in  order  to  avoid  loss  of  nitrogen  in  consequence  of  a  high 
chlorine  content.  This  determination  is  most  significant  where 
oxidized  effluents  are  concerned,  and  the  nitrate  figures  are  to 
be  interpreted  as  being  at  least  the  recorded  values.  The  deter- 
mination of  both  nitrite  and  nitrate  nitrogen  was  preceded  by 
clarification  of  the  samples  by  alum. 

Temperature  was  recorded  in  degrees  centigrade. 

Dissolved  Oxygen  was  determined  by  the  use  of  the  Hale  and 
Melia  modification  of  the  Winkler  process. 

Relative  Stability  was  determined  in  bottles  containing  250  c.c. 
of  the  sample,  when  full,  which  were  stoppered  with  corks,  after 
adding  y2  c.c.  of  a  0.05  per  cent,  aqueous  solution  of  methylene 
blue.  The  bottles  were  incubated  at  the  laboratory  room  tem- 
perature, and  the  time  of  decolorization  noted.  The  relative 
stability  number,  or  value  in  terms  of  the  per  cent,  stable,  was 
obtained  from  the  table  in  Standard  Methods,  A.  P.  H.  A. 

Oxygen  Demand  (Biochemical),  or  more  simply  Demand,  is 
the  amount  of  dissolved  oxygen  in  parts  per  million  parts  of  the 
sewage,  which  sample  of  sewage,  or  treated  effluent,  will  take  up 
from  thoroly  aerated  clean  water,  when  incubated  in  a  glass- 
stoppered  bottle,  at  room  or  standard  temperature  for  five  days. 
It  is  assumed  that  it  measures  the  amount  of  dilution  necessary 
to  prevent  a  nuisance  when  sewage  is  discharged  into  a  body  of 


12  Brooklyn,  N.  Y. 

water,  and  also  to  approximate  the  condition  of  a  stream,  if 
re-aeration  did  not  take  place.  The  determination  was  made  ac- 
cording to  Standard  Methods,  A.  P.  H.  A. 

Local  Conditions 

The  Twenty-sixth  Ward  of  the  boro  of  Brooklyn  constitutes 
the  extreme  easterly  portion  of  the  boro.  The  population  (1918) 
was  estimated  from  police  and  school  records  to  be  220,000.  The 
surface  slopes  from  high  lands  along  the  northerly  boundary,  at 
first  with  short  but  sharp  grades,  into  a  gently  sloping  plain,  which 
constitutes  about  85  per  cent,  of  the  area.  This  has  resulted  in 
the  design  of  a  sewerage  system  with  very  flat  grades.  The 
drainage  and  sewage  are  discharged  into  the  waters  of  Jamaica 
Bay. 

The  hourly  rate  of  flow  in  dry  weather,  based  upon  data  de- 
termined by  weir  measurements  in  the  outfall,  varies  at  present 
(1918)  from  about  18,000,000  to  27,000,000  gallons  per  day. 
The  storm  flow  from  the  area  at  present  provided  with  sewers 
ranges  up  to  1,000  cubic  feet  per  second,  altho  the  ordinary 
storm  does  not  give  more  than  300  to  500  cubic  feet  per  second. 
The  dry  weather  flow  is  mainly  of  a  domestic  character.  The 
suspended  matters  change  widely  in  quantity  at  different  seasons, 
and  at  different  hours  of  the  day.  The  increase  of  population 
occupying  the  drainage  area  is  notable  and  of  interest  in  show- 
ing how  rapidly  New  York  suburban  districts  become  urban,  as 
may  be  seen  from  Table  I. 

The  sewerage  system  was  mainly  installed  between  the  years 
1890  and  1896  and  includes  a  chemical-precipitation  sewage-treat- 
ment plant,  designed  in  1888  and  intended  to  take  care  of  a 
maximum  population  of  35,000,  which,  at  the  time,  was  consid- 
ered ample  provision  for  the  future.  This  plant  was  completed 
in  1896,  when  the  population  had  already  reached  about  60,000. 
It,  therefore,  was  inadequate  from  the  beginning  of  its  operation, 
altho  for  several  years  it  rendered  fairly  good  service. 

The  sewage  passing  thru  the  plant  flows  thru  fixed  screens 
and  narrow  settling  tanks,  with  considerably  velocity,  to  a  cen- 
tral well,  from  which  it  is  pumped  into  the  outfall  sewer.  The 
pumps  have  a  nominal  capacity  of  about  20,000,000  gallons  per 
day,  but  on  account  of  depreciation  are  incapable  of  pumping 
more  than  half  this  quantity  at  present. 

The  twin  outlet  sewers,  which  are  combined  sewers  with  very 
fiat  grades,  terminate  at  the  location  of  the  plant  in  a  silt  basin, 
which,  in  dry  weather,  serves  as  a  grit  chamber  for  the  sanitary 
sewage.  This  basin,  which  is  covered,  is  80  ft.  by  60  ft.  in  plan, 
and  is  9  ft.  deep,  acts  as  a  trap  to  divert  the  sewage  in  dry 
weather  into  the  treatment  plant.  The  basin  is  provided  with  a 
hisfh  level  overflow  for  storm  water,  and  the  line  of  the  sewer  is 


Sewage  Treatment  Experiments  13 

continued  by  means  of  an  outfall  flume,  beginning  at  the  high- 
level  overflow  mentioned  and  extending  from  the  basin  to  Jamaica 
Bay.  The  outfall  flume  is  a  single  channel  section,  26  feet  wide ; 
its  invert  grade  begins  at  the  basin  3.40  feet  above  the  invert 
grades  of  the  twin  sewers  and,  as  it  is  inadequate  in  storms  of 
much  severity,  it  backs  up  the  flow  in  the  twin  sewers  and  causes 
flooding.  In  dry  weather  the  sewage  which  cannot  be  taken  care 
of  by  the  existing  treatment  plant  overflows  thru  the  storm  outlet 
and  discharges  into  the  bay  without  treatment. 

A  passageway..  48.  inches  in  diameter,  is  provided  from  the 
grit  chamber  to  the  existing  treatment  plant,  with  the  invert  at 
the  floor  elevation  of  the  grit  chamber,  for  carrying  the  dry- 
weather  flow  into  the  plant,  where,  after  receiving  its  charge  of 
lime,  and  having  passed  thru  the  screens  and  tanks,  it  flows  into 
the  central  pump  well.  From  the  well  the  treated  sewage  is 
pumped  against  an  average  head  of  20  feet  into  the  outfall  flume 
at  a  point  about  100  feet  downstream  from  the  silt  basin.  During 
storms  the  by-pass  valves  are  closed  and  the  entire  flow  dis- 
charges directly  from  the  grit  chamber  thru  the  outfall  flume 
to  the  bay. 

During  a  moderate  rainfall  the  volume  of  flow  carried  off  by 
the  sewers  is  about  ten  times  the  ordinary  volume  in  dry  weather. 
To  make  matters  worse,  the  existing  sewers  are  not  of  modern 
design  and  do  not  adapt  themselves  well  to  this  double  service  and 
are  usually  too  small  for  present  conditions  of  storm  service. 
They  are,  however,  much  too  large  for  efficient  operation  in  dry 
weather,  and  this  is  especially  the  case  in  the  outfall  sewers, 
which  on  account  of  low  street  grades  and  lack  of  sufficient 
cover,  are  flat  and  broad,  and  blanketed  by  the  tide  at  their  outlet. 

This  condition  results  in  making  the  sewers  elongated  settling 
tanks  as  they  approach  the  outlet  of  the  system,  and  the  dry 
weather  flow  passes  thru  them  with  progressively  falling  velocity 
from  the  lateral  sewers  to  the  outlet ;  they  are  all,  therefore, 
sewers  of  deposit  in  dry  weather  and  would  cause  great  trouble 
from  smells  as  well  as  from  clogging  with  banks  of  sedimented 
solids,  if  it  were  not  that  the  periodical  storms  flush  them  out, 
carrying  the  matters  that  have  collected  in  dry  weather  with  the 
rush  of  the  flood  wave  into  Jamaica  Bay.  The  first  part  of  the 
storm  flow  is,  therefore,  exceedingly  foul,  much  more  so  than  the 
ordinary  dry-weather  flow. 

Measurement  of  Seivage. 

The  measurement  of  the  total  flow  of  sewage  discharged  by 
the  main  outfall  sewer  in  dry  weather,  and  its  hourly  and  daily 
variations  was  obtained  by  means  of  a  knife-edge  weir  installed 
in  the  outfall  flume.  It  was  sharp  crested  with  end  contractions 
suppressed.     The  crest  length  was  25.84  ft.  and  the  height  2.17 


14  Brooklyn,  N.  Y. 

ft.  The  horizontal  knife-edge  was  installed  by  means  of  a  wye 
level ;  the  adjustments  were  made  from  slotted  bolt  holes,  and 
finally  by  filing  off  very  small  irregularities. 

The  device  for  recording  the  heads  of  flow  on  the  weir  con- 
sisted of  an  automatic  water  stage  register,  with  a  specially  de- 
signed apparatus  for  magnifying  the  gage  heights  placed  in  a 
Bazin  pit  installed  just  outside  of  the  channel,  and  16  feet  up 
stream  from  the  crest  of  the  weir.  The  pit  was  connected  with 
the  flume  by  means  of  an  iron  pipe  3  feet  long,  laid  on  the  floor, 
and  perpendicular  to  the  side  of  the  approach  channel  to  the  weir. 

The  zero  of  the  register  was  determined  by  means  of  a  hook 
gage  and  wye  level.  The  correction  for  slack  motion  of  the 
register  was  determined  by  hook  gage  to  be  ^  of  1  per  cent. 

A  similar  knife-edge  weir,  with  end  contractions  suppressed, 
with  a  crest  8  feet  in  horizontal  length,  was  used  to  measure  efflu- 
ent from  the  Riensch-Wurl  Screens. 

This  weir  was  placed  in  a  specially  prepared  flume  located 
parallel  to  and  alongside  of  the  main  outfall  flume,  the  flow  re- 
entering the  main  flume  several  hundred  feet  below  the  experi- 
ment station. 

The  details  of  this  weir  are  similar  to  those  of  the  larger  weir. 

In  computing  the  discharge  over  these  weirs  the  formulae  of 
Bazin,  Fteley  and  Stearns,  Francis,  and  Hamilton  Smith,  were 
employed.  The  formula  of  Bazin  was  selected  as  probably  the 
most  applicable  to  the  kind  of  weir  used  under  the  observed  con- 
ditions. Taking  Bazin  at  100  per  cent  we  have  the  following 
comparative  values : 

Bazin .100    % 

Fteley  and  Stearns.. 96.2% 

Francis  95.6% 

Hamilton  Smith 95.  % 

Calibrated  Orifices. — For  the  measurement  of  sewage  delivered 
to  the  various  units  of  the  experimental  plant,  calibrated  orifices, 
discharging  under  a  constant  head,  were  selected  as  the  most  sat- 
isfactory, as  errors  would  be  less  than  would  be  the  case  with 
weirs.  In  the  use  of  such  orifices,  the  calibration  could  be 
checked  experimentally  with  the  apparatus  in  place  by  discharg- 
ing into  a  measuring  tank,  or  into  the  unit  which  the  given  orifice 
was  designed  to  supply  with  sewage.  The  latter  was  found  to 
be  a  satisfactory  method.  The  calibration  was  performed  against 
the  tank  which  was  to  be  controlled  by  the  orifice,  and  the  head 
was  held  by  allowing  a  slight  excess  to  waste  over  a  weir  above 
the  orifice. 

The  orifices  originally  designed  were  made  of  bronze ;  they 
were  adjustable,  and  provided  with  scales  for  setting  the  size  of 
the  opening  for  various  rates  of  discharge.     To  measure  small 


Sewage  Treatment  Experiments 


15 


rates  of  flow,  orifices  were  made  in  copper  plate  in  the  machine 
shop  of  the  treatment  plant,  which  gave  entire  satisfaction. 

The  accurate  measurement  of  sewage  is  at  best  a  difficult  mat- 
ter. The  suspended  material  will  affect  almost  any  form  of  ap- 
paratus used  for  the  purpose ;  orifices  not  excepted.  However, 
with  regular  and  frequent  cleaning,  the  types  of  orifices  adopted 
were  satisfactory  and  gave  results  that  were  well  within  the  ac- 
curacy of  the  chemical  tests  and  their  interpretation. 

Measurement  of  Compressed  Air. — On  account  of  the  volume 
of  air  to  be  metered,  and  the  pressure,  no  form  of  available  gas 
meter  was  found  fully  to  answer  the  requirements ;  therefore, 
several  forms  of  Venturi  meters  were  studied  and  a  meter  was 
developed,  which  worked  very  well.  The  contract  to  make  and 
install  these  meters,  and  to  test  them  in  place,  was  awarded  to 
Messrs.  Wallace  &  Tiernan,  of  New  York. 

The  quantity  of  compressed  air  that  would  be  required  in 
sewage  aeration  was  unknown,  at  the  beginning  of  the  experi- 
ments, and  it  was  therefore  necessary  to  provide  for  a  wide  range 
in  measuring  capacity.  This  was  accomplished  by  having  several 
interchangeable  throats  of  different  sizes  for  each  Venturi  meter, 
and  a  specific  manometer  scale  for  each  throat. 

Three  sets  of  meters  were  obtained,  each  of  which  was  stand- 
ardized as  to  measuring  capacity,  against  an  operating  head  of 
40  lbs.  air  pressure,  and  calibration  scales  were  worked  out  for 
pressures  from  a  minimum  of  5  lb.  to  a  maximum  of  40  lb.  per 

AIR    VENTURI    METER 


Fig.  4. 


16 


Brooklyn,  N.  Y. 


square  inch,  in  order  that  by  exchanging  scales  any  pressure 
within  these  limits  might  be  used.  The  measuring  capacity  of 
each  set  of  meters  under  the  40  lb.  pressure  standard  was  as 
follows : 

Set  No.  1  capacity  20  to  150  cu.  ft.  per  minute. 

Set  No.  2  capacity    5  to    20  cu.  ft.  per  minute. 

Set  No.  3  capacity     1  to    30  cu.  ft.  per  minute. 

The  meters  were  each  assembled  in  cylinders  of  the  same  size, 
so  that  any  one  of  them  could  be  installed  in  the  body  of  an  or- 
dinary 3-inch  check  valve,  put  in  the  air  line.  There  were  three 
Venturi  tubes  complete,  supplied  for  each  meter ;  two  of  these 
were  of  small  capacity,  of  which  each  had  two  interchangeable 
throats,  while  the  third  was  larger  and  had  but  one  throat. 

Referring  to  the  drawing,  Fig.  4,  air  passes  along  the  line  indi- 
cated by  the  arrows,  thru  the  throat  and  out  thru  the  air  line. 

The  manometer  used  was  of  special  design.  The  cross  section 
of  the  oil  reservoir  was  made  many  times  greater  than  that  of  the 
manometer  tube,  so  that  the  zero  point  was  practically  unaffected 
thru  the  range  of  the  scale.  The  upper  end  of  the  manometer 
tube  was  connected  with  the  Venturi  throat  chamber,  and  had  an 
upper  reservoir  interposed,  of  slightly  greater  capacity  than  that 
of  the  lower  reservoir.  This  was  provided  to  prevent  the  blowing 
of  any  oil  into  the  pipe  line  by  sudden  rushes  of  air. 

The  calibration  of   these  meters  was  an  interesting  problem. 

CALIBRATION        BOX 


s= 

/ 

" 

Fig.  5. 


On  account  of  the  quantity  of  air  it  was  impracticable  to  use  a 
gasometer.  Therefore  a  calibrating  box,  consisting  of  a  special 
multiple-orifice  box,  was  designed  to  receive  and  measure  the  air 


Sewage  Treatment  Experiments 


17 


passing  thru  the  meter.  This  box  is  shown  in  Fig.  5.  The  top 
of  the  box  was  a  metal  plate  having  in  all  150  orifices,  each  ex- 
actly equal  in  size  and  all  made  with  the  same  die,  and  equally 
spaced.  A  cover  plate  was  provided,  so  designed  that  any  num- 
ber of  orifices  from  1  to  150  might  be  left  open.  The  orifices 
were  each  computed  and  made  of  proper  size  to  discharge  1  cu. 
ft.  of  air  per  minute  under  a  head  of  10  inches  of  water  pressure. 
A  perforated  metal  screen  was  placed  diagonally  across  the  box 
to  prevent  eddy  currents.  Preliminary  readings  were  made  for 
rating  this  work,  with  a  small  gasometer,  to  determine  the  air 
flows  from  different  orifices  and  from  different  groups  of  orifices 
to  see  if  there  was  any  variation  because  of  location  or  number 
of  those  discharging.  The  flows  were  found  to  agree  within  1/5 
of  1  per  cent. 

In  calibrating  a  meter,  the  box  was  connected  with  the  air 
line  thru  the  meter,  and  a  certain  number  of  orifices  uncovered 
on  the  top  of  the  box,  a  stop-valve  being  opened  until  the  desired 
head  was  shown  on  the  box  manometer. 

Knowing  the  discharge  for  this  number  of  orifices,  and  the 
corresponding  pressure,  it  was  a  simple  matter  to  mark  on  the 
Venturi  manometer  scale  the  flow  corresponding  to  the  height  of 
the  liquid. 

After  having  determined  several  of  these  points,  the  scale 
was  graduated  in  cubic  feet. 

Distribution  Control. — In  order  that  the  sewage  coming  from 
the  pumps  might  pass  by  gravity  to  every  unit,  and  that  the 
impulse  imparted  by  the  pump  might  be  removed,  a  quieting  and 
distributing  tank  was  provided,  thru  which  the  entire  plant  could 
be  supplied  except  the  R.  W.  fine  screens. 


PLAN 


•*C>JUST*&U  WFICt  . 


QUIETING    TANK 
SECTION  A-A 


SECTION  B-B 


CKSM  una&t  aimj  I 


Fig.  6. 


18  Brooklyn,  N.  Y. 

Quieting  Tank. — The  quieting  tank  (Fig.  6)  was  designed  to 
maintain  a  constant  head  in  the  supply,  at  elevation  33.42  feet. 
A  platform  around  the  tank  was  provided  for  convenient  inspec- 
tion and  operation,  and  connected  by  means  of  a  bridge  with  the 
tops,  or  "decks",  of  the  three  Imhoff  tanks. 

Obtaining  Seivage  for  the  Experiments 

When  the  work  was  commenced  on  the  design  of  the  experi- 
mental plant  in  1912,  one  of  the  first  problems  presented  was 
how  to  obtain  sewage  from  the  large  sewers  of  fair  average 
strength  and  condition. 

The  following  local  conditions  prevailed :  The  passages  of 
the  large  twin  sewer,  each  12  ft.  6  in.  wide  by  9  ft.  high,  entered 
the  grit  chamber  with  their  inverts  at  — ■  2.40,  datum  being  mean 
high  water.  The  surface  of  the  flow  of  sewage  in  the  sewers  and 
grit  chamber  at  this  point  averaged  about  elevation  -\-  2.37  for 
dry-weather  flow  sewage,  but  was  subject  to  changes  of  level 
during  the  day  between  elevation  -\-  1.75  and  -|-  3.00. 

The  surface  velocity  of  the  sewage  as  it  entered  the  grit 
chamber  ranged  from  about  half  a  foot  to  more  than  a  foot  per 
second,  under  different  conditions  of  flow. 

The  sewers  and  grit  chamber  acted  to  some  extent  as  a  sedi- 
mentation tank,  sewage  solids,  as  well  as  grit,  being  deposited 
a^  all  times  during  dry  weather  in  greater  or  less  quantities.  The 
inverts  of  the  larger  sewers  of  the  Twenty-sixth  Ward  are  shal- 
low and  wide,  and  the  grades  too  flat,  as  a  rule,  to  afford  suffi- 
cient velocity  of  flow  to  prevent  considerable  of  the  suspended 
solids  in  the  sewage  from  settling  and  forming  deposits,  which 
during  storms  are  picked  up  by  the  high  velocity  of  flow  and 
flushed  out  into  Jamaica  Bay.  During  the  first  rush  of  the  storm 
the  discharge  is  even  more  foul  than  the  ordinary  discharge  of 
sewage  in  dry  weather.  Following  this,  however,  after  the  flush- 
ing out  of  these  deposits  has  been  completed,  the  storm  flow 
rapidly  improves  in  quality  and  soon  becomes  mere  dirty  water, 
which  is  incapable  of  causing  a  nuisance,  and  does  not  require 
treatment. 

The  by-pass,  provided  to  carry  the  dry- weather  flow  from  the 
grit  chamber  of  the  treatment  plant,  is  given  off  from  the  lower 
portion  of  the  chamber,  the  bottom  of  which  agrees  with  the 
invert  of  the  by-pass,  and  is  elevation  — 3.00.  Since  the  by-pass 
is  48  in.  in  diameter,  the  crown  of  its  arch  is  elevation  -f-  1.00.  It 
enters  the  treatment  plant  with  its  gates  at  invert  elevation  —  3.50, 
which  is  above  the  water  line  in  the  receiving  tanks  of  the  plant, 
so  that  sewage  enters  with  a  free  flow. 

The  name  "grit  or  silt  chamber"  is  really  a  misnomer,  for 
most  of  the  grit  and  silt  settles  from  the  sewage  in  the  large 
sewers  before  the  chamber  is  reached. 


Sewage  Treatment  Experiments  19 

All  of  the  storm  flow  and  about  two-thirds  of  the  dry  weather 
flow,  which  cannot  be  admitted  to  the  treatment  plant,  discharges 
from  the  grit  chamber  at  elevation  -[-1.00  into  the  outfall  flume 
which  runs  3,230  feet  to  the  bay. 

It  was  observed  that  the  sewage  passing  into  the  treatment 
plant,  thru  the  by-pass,  gave  a  more  consistent  average  proportion 
of  the  total  and  suspended  solids,  than  that  which  passed  into  the 
bay  thru  the  outfall  sewer,  which,  on  the  average,  was  a  weaker 
sewage,  altho  at  times  it  was  much  stronger,  and  carried  more 
floating  matter,  as  well  as  settleable  solids,  but  was  subject  to 
greater  variation  in  suspensa,  as  well  as  volume  of  flow.  These 
conditions  indicated  that  the  mo-st  appropriate  sewage  for  study 
in  the  experimental  work  was  that  entering  the  treatment  plant, 
as  near  the  outlet  end  of  the  by-pass  as  practicable.  For,  altho 
this  sewage  was  slightly  stronger  than  the  average  of  the  entire 
flow,  this  would  give  results  on  the  safe  side ;  while  the  other  flow 
was  weaker  than  the  average,  and  would,  therefore,  give  more 
fallacious  results. 

TABLE  I 

Population  of  the  26th  Ward  of  Brooklyn,  N.  Y. 

Increase  of  Population, 
26th  Ward,  Brooklyn. 
Year  Population  to  nearest  1,000 

1880  13,000 

1890  30,000 

1900  66,000 

1905  94,000 

1910  178,000 

1914  200,000 

1918  : 220,000 

TABLE  II 

Daily  and  Hourly  Flow  of  Sewage 

Dry-weather  flow  of  sewage  contributed  by  220,000  peo- 
ple REACHING  THE  OUTLET  IN  HENDRIX  STREET,  BASED  UPON 
DATA  OBTAINED  BY  WEIR  MEASUREMENTS,  REDUCED  TO  HOURLY  PER- 
CENTAGE OF  FLOW  PER  CAPITA  :  MAXIMUM  DAILY  RATE  PER  CAP- 
ITA 121  gallons.  Minimum  rate  81  gallons.  Prepared  for 
January  1,  1918. 

Hour  Pet.  of  max.  flow     Cu.  ft.  per  sec.        Rate  in  M.g.d. 

12  a.  m 78.5  32.49  21 

1  a.  m 74.4  30.92  20 

2  a.  m 71.  29.40  19 

3  a.  m 67.8  28.62  18.5 

4  a.  m 67.  27.85  18 

5  a.  m.  min 66.1  27.07  17.6 

6  a.  m 67.8  27.85  18 

7  a.  m 72.8  30.17  19.5 

8  a.  m 81.  33.65  21.75 


20  Brooklyn,  N.  Y. 

Hour  Pet.  of  max.  flow     Cu.  ft.  per  sec.      Rate  in  M.  g.  d. 

9  a.  m 87.6                      36.36  23.5 

10  a.  m 91.7                       37.90  24.5 

11  a.  m 93.4                       38.68  25 

12  p.  m 96.7                       40.23  .                  26 

1  p.  m 99.2                       41.00  26.5 

2  p.  m 99.2                       41.00  26.5 

3  p.  m.  max 100.                         41.29  26.62 

4  p.  m 97.5                      40.61  26.25 

5  p.  m 96.9                      39.84  25.75 

6  p.  m 94.2                      39.06  25.25 

7  p.  m 89.2                      37.13  24 

8  p.  m 85.9                      35.59  23 

9  p.  m 84.3                      34.81  22.5 

10  p.  m 84.3                      34.81  22.5 

11  p.  m 82.6                      34.42  22.25 

12  p.  m 78.5                       32.49  21 

The  weekly  cycle  of  daily  and  hourly  per  capita  flow  of  sewage 
is  shown  by  Table  III,  as  well  as  the  quantity  of  flow  for  each 
day  of  the  week.    The  figures  are  averages. 


TABLE  III 

Per  Capita  Flozv  of  Sewage  in  Gallons  for  Every  Hour  of  the 

Day  and  Week 

1914-1915 

Sun.      Mon.  Tues.  Wed.     Thurs.      Fri.  Sat. 

Hour                                    gal.         gal.  gal.  gal.  gal.         gal.  gal. 

5  a  .m 3.3          3.4  3.6  3.2  3.4  3.6  3.0 

6  a.  m 3.3          3.6  3.8  3.3  3.5  3.6  3.2 

7  a.  m 3.4          3.8  4.1  3.6  3.6  3.6  3.5 

8  a.  m 3.9          4.2  4.4  4.3  4.2  4.1  3.9 

9  a.  m 4.5          4.8  4.8  4.5  4.8  4.5  4.3 

10  a.  m 4.8          5.8  5.0  4.6  5.0  4.7  4.4 

11  a.  m 5.0          5.1  4.8  4.7  5.0  5.0  4.4 

12  a.  m 5.1          5.4  5.0  4.7  5.0  5.0  4.7 

1  p.  m 5.0          5.5  5.0  4.7  5.1  5.0  4.8 

2  p.  m 4.8          5.2  4.8  4.8  5.3  5.1  5.0 

3  p.  m 4.8          5.1  5.1  4.8  5.3  5.2  4.0 

4  p.  m 4.7          4.8  5.1  4.8  5.0  4.4  4.8 

5  p.  m 4.5          4.7  5.0  4.7  5.0  4.3  4.7 

6  p.  m 4.4          4.6  4.7  4.6  4.8  4.3  4.5 

7  p.  m 4.2          4.5  4.5  4.6  4.7  4.2  4.4 

8  p.  m 4.2          4.5  4.5  4.4  4.5  4.2  4.4 

9  p.  m 4.1          4.5  4.4  4.4  4.1  4.1  4.2 

10  p.  m 4.1          4.4  4.3  4.4  4.4  4.1  4.1 

11  p.  m 4.1          4.3  4.2  4.3  4.3  4.1  3.8 

12  p.  m 3.9          4.2  3.9  3.9  4.2  3.9  3.7 

1  a.  m 3.7          3.9  3.9  3.7  3.7  3.9  3.5 

2  a.  m 3.5          3.6  3.8  3.5  3.5  3.8  3  3 

3  a.  m 3.4          3.5  3.7  3.3  3.3  3.6  3.3 

4  a.  m 3.3          3.4  3.6  3.2  3.3  3.6  3.1 

Totals  100.0      106.0  106.0  101.0  105.0      101.9  97.8 

Mean  flow  for  week,   102. S  gallons  per  capita. 


Sewage  Treatment  Experiments  21 

The  Storm  Water  Sewage 

The  effect  of  storms  on  the  quality  of  the  sewage  was  always 
greatly  to  increase  the  matters  in  suspension  during  the  first 
period  of  the  storm.  Where  the  storm  was  of  short  duration, 
this  increase  continued  thruout  the  storm,  but  if  the  duration 
was  continued  over  several  hours,  a  marked  improvement  took 
place  in  the  flow. 

This  condition,  as  already  mentioned,  was  probably  due  to  the 
large  size  of  the  main  sewers,  which  are  on  the  combined  plan, 
and  have  very  flat  grades  thru  much  of  their  extent,  and  shallow 
inverts,  into  which  considerable  settling  matter  falls  in  dry 
weather,  being  flushed  out  by  the  flood  wave  of  the  storm. 

Two  storms,  of  ordinary  severity  such  as  frequently  occur 
at  this  place,  and  a  rather  high  rate  of  precipitation,  may  be  given 
here  in  illustration  of  the  phenomena  attending  upon  the  flow  of 
storm  sewage,  during  the  usual  summer  shower. 

The  first  storm  referred  to  ("A"  in  Table  IV)  took  place  on 
May  27,  1914.  The  total  rainfall  recorded  was  .15  inches,  of 
which  .10  fell  in  the  first  half  hour.  The  second,  ("B"  in  the 
table)  occurred  on  August  21  of  the  same  year.  The  rainfall, 
according  to  the  gage,  was  1.0  inch  in  all,  0.9  inch  falling  in 
the  first  half-hour. 

The  following  phenomena  were  common  to  both  storms.  After 
a  lapse  of  from  twenty-five  minutes  to  an  hour  from  the  begin- 
ning of  rainfall,  the  sewage  became  very  foul,  as  shown  by  the 
remarkable  leap  in  suspended  solids.  The  persistence  of  this 
abnormal  content  was  only  about  an  hour  in  the  heavier  storm, 
and  about  two  hours  in  the  lighter  one.  The  presence  of  consid- 
erable quantity  of  gritty  street  washings  is  indicated  by  a  marked 
drop  in  the  percentage  of  volatile  matter  in  the  suspensa.  In 
the  storm  of  May  27,  this  drop  took  place  some  time  later  than 
the  time  of  maximum  suspended  matter,  but  in  the  storm  of 
August  21,  the  street  wash  appeared  to  come  coincidently  with 
the  outflush  of  sludge  from  the  inverts  of  the  trunk  sewers.  The 
accompanying  Table  IV  gives  the  figures : 

TABLE  IV 

Hrs.  after  Settling 

beginning  Matter  Suspended  Solids  Dissolved 

of  rain  c.c.l.  Total  p. p.m.  Volatile  %         Oxygen  p.p.m. 

A  B                A                B  A  B             A  B 

Date                       5-27  8-21  5-27  8-21  5-27  8-21  5-27  8-21 

0     2.7  3.2              226              252  74  76              .6  .7 

Va 1.5  3.0              208              254  78  75              .5  .6 

Vi 2.1  6.2              248              412  73  77              .4  0 

54 2.5  .  16.5              250  2250  72  40              .4  0 

1     29.0  12.8  1880  1976  72  41  0  0 

W2 2.7  4.0  588  820  35  27  0  0 

2     5.4  2.5  1126  480  28  30  .1            1.9 

2J4 5.5  2.6  1096  480  29  38  .6            1.1 

3     4.2             494           39  ....  .7 

4     2.2             204           47  ....  2.5 

5     9  118  63  ....  1.9 

6     5  84  52  ....  2.4 


22 


Brooklyn,  N.  Y. 


The  most  important  characteristics  of  the  dry-weather  flow 
sewage  are  exhibited  by  Table  V,  which  for  the  data  given  covers 
the  period  of  the  experimental  work  of  the  station,  upon  which 
the  report  is  based.  It  should  be  noted  that  the  figures  given 
are  averages,  and  as  such  do  not  show  either  extreme  of  the  con- 
ditions. 

TABLE  V 
General  Characteristics  of  26th  Ward  sewage 

From  Monthly  Averages  for  the  Period  of  Experiments 

Oxygen 

Suspended  Solids — Demand 

Month  Temp.         Diss.       Oxygen       Total       Volatile    Non-Vol.      Cone       Biochem 

°C  p.p.m.       %  sat.       p. p.m.       p. p.m.       p. p.m.        c.c.l.         p.p.m. 


Jan 11.0  4.5  41  175  140  35  2.1  209 

Feb 10.0  4.5  40  172  134  38  2.0  262 

March    11.6  3.6  33  192  142  50  1.8  220 

April    14.7  2.8  27  162  124  38  1.7  195 

May     18.2  1.9  20  178  136  42  1.9  213 

June     20.9  1.2  13  153  118  35  1.6  250 

July     26.0  0.8  10  163  118  45  1.7  202 

Aug 23.8  0.6  7  146  112  34  1.7  203 

Sept 22.1  0.5  6  168  136  32  2.1  237 

Oct 17.5  1.9  20  146  108  38  1.8  205 

Nov 13.9  3.8  37  160  132  28  1.7  254 

Dec 10.8  3.8  34  203  155  48  2.6  223 

Average     16.7  168  129  39  1.9  223 

Dec-Mar     10.8  4.1  37  186  143  43  2.1  228 

Apr.-June    18.0  2.0  21  164  126  38  1.7  219 

July-Sept 24.0  0.6  7  159  122  37  1.8  214 

Oct. -Nov 16.C  2.9  29  153  120  33  1.8  229 


Seasonal  variations  by  averages 


Month 
&  Year 


TABLE  VI 
Showing  Oxygen  Relations  of  Sewage  for  One  Year 

Oxygen   Consumed  in  30   Minutes   Digestion 


Susp'd  Solids       Nitrogen  as 


Oxygen 
Consumed 


Diss. 
Oxygen" 


Temp.  C 


1914 


Is 


H  & 


■a  s 

.ti  d 

Zd 


.t;  d 
Zd 


3S 

C  D. 

id 


iS  d 


April 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

1915 

Jan. 

Feb. 


126 
164 
125 
152 
128 
164 
127 
158 
201 


175 
219 


March     203 


129 
96 
97 
101 
130 
101 
126 
155 


142 
168 
155 


.09 
.37 
.21 
.07 
.13 
.28 
.37 

.33 
.13 
.31 


.31 
.09 
.02 
.01 
.05 
.05 
.07 
.14 
.13 

.13 
.13 
.16 


53 
60 
48 
56 
53 
58 
51 
51 
71 

74 
82 

75 


35 
44 
38 
37 
35 
39 
44 


2.2 
1.5 
1.8 
1.2 

1.2 
2.2 
1.9 

2.0 


2.6 
2.5 
1.4 


22 
17 

21 
14 
14 
24 
19 
18 


13.7 
18.3 
21.1 
22.3 
23.9 
21.4 
19.4 
15.9 
12.0 


11.6 
11.0 
13.3 


*  Comparative    dissolved    oxygen    values    during    an    average    day,    by    two-hour    periods. 
Lowest  hourly  value  assumed  to  equal    1,000.     Twenty-sixth  Ward  Sewage: 


Midnight  1.100 

2  a.  m 1.100 

4  a.  m 1.205 

6  a.   m 1.300 

8  a.  m 1.545  max. 

10  a.  m 1.478 

Noon   1.363 


2  p.  m 1.182 

4  p.  m 1.341 

6  p.  m 1.000  min. 

8  p.  in 1.100 

10  p.  m 1.114 

Midnight  1.100 


Sewage  Treatment  Experiments  23 

TABLE  VII 
Average  Nitrogen  Contents  of  Sewage 

Free   ammonia 32.00  p.p.m. 

Organic  nitrogen: 

Total     27.00  p.p.m. 

Dissolved    18.00  p.p.m. 

Nitrites     0.26  p.p.m. 

Nitrates    0.00  p.p.m. 

Nitrites  and  nitrates  are  frequently  absent,  and  the  sewage  was 
frequently  septic  during  the  summer  months,  but  very  seldom 
during  the  other  seasons. 

TABLE  VIII 
Daily  cycle  of  sewage,  showing  hourly  changes  in  strength 

Suspended  Solids  Oxygen  Consumed* 

Settling  Total  Volatile  Total  Diss. 

**c.c.  perl,      p.p.m.  p.p.m.  p.p.m.  p.p.m. 

Midnight     1.6  135  111  58  42 

2   a.   m 1.4  168  137  58  39 

4  a.  m 1.8  128  107  51  36 

6   a.   m 1.0  93  74  39  27 

8  a.  m 1.4  88  64  36  25 

10  a.   m 2.0  146  110  67  46 

Noon    2.0  196  159  81  48 

2  p.  m 1.7  177  139  72  47 

4  p.   m 2.3  190  147  84  51 

6  p.  m 2.1  187  147  96  59 

8  p.   m 2.2  183  140  88  56 

10  p.  m 1.5  149  115  70  48 

Midnight 1.6  135  111  58  42 

*  Oxygen  consumed  in  thirty  minutes  at   100°C.     Diss,  is  from  filtered  sample. 
**  The    settling    suspended    solids    in    the   first    column    are    obtained    by    means    of    the 
Imhoff  cone. 

One  of  the  notable  features  of  the  local  sewage  is  the  large 
proportion  of  colloidal  suspensa  in  very  finely  divided  condition, 
which  settles  very  slowly  if  at  all.  The  different  tanks  removed  a 
satisfactory  percentage  of  settleable  solids,  but  to  a  large  extent 
this  non-settleable  material  remained  in  the  sewage. 

That  portion  of  this  fine  material  which  disappeared  in  passage 
thru  the  tanks  probably  did  not  settle  but  was  dissolved. 

Experiments  in  Imhoff  settling  cones  were  made  to  determine 
the  average  rate  of  sedimentation.  The  volume  of  material  in 
the  apex  of  the  cone  was  read  after  the  expiration  of  varying 
intervals.  The  following  results,  obtained  from  an  extensive 
series  of  observations,  are  averages  of  the  quantity  of  settling 
matter  in  cubic  centimeters  per  liter,  and,  assuming  the  average 
amount  settled  in  two  hours  equal  to  100  per  cent.,  the  percent- 
ages, for  shorter  periods  are  given. 

Time                                                Settlings  Per  Cent. 

Minutes                                          c.c.  per  liter  Settled 

5  0.629  33.1 

10  1.109  58.3 

15  1.188  62.5 

30 1.359  71.5 

45  1.480  78.0 

60  1.490  78.4 

120  1.900  100. 


24  Brooklyn,  N.  Y. 

The  above  matters,  settling  per  liter,  were  equivalent  only  to 
about  69  p.  p.  m.  of  the  sewage,  which  contained  162  p.  p.  m. 
of  suspended  solids.  In  other  words,  1.90  c.c.  per  liter  represents 
only  about  43  per  cent,  of  the  suspended  solids.  The  remainder 
would  not  settle  in  a  two-hour  period,  and  even  in  six  hours  but 
a  small  portion  of  it  would  settle.  It  is  obvious,  therefore,  that 
the  tanks  would  not  remove  in  two  hours  a  greater  proportion 
than  could  be  removed  in  the  cones.  In  the  results  of  removal 
effected  by  the  tanks  given  in  the  tables  which  follow,  this  should 
be  understood.  The  tabulated  figures  refer  to  the  constituents 
observed  in  the  sewage  and  in  the  effluents'.  In  Table  XIV  the 
percent  of  removal  is  also  shown. 

The  removal  of  43  per  cent,  of  total  suspended  solids  might  be 
stated  as  far  as  the  tank  is  concerned,  as  equal  to  a  removal 
of  100  per  cent.,  because  it  is  quite  clear  that  the  tanks  cannot 
remove  in  two  hours  any  more  solids  by  sedimentation  than  a 
cone  can  in  the  same  interval  of  time. 

That  the  Imhoff  cone  should  be  a  more  efficient  remover  of 
suspended  solids  than  the  tanks  is  not  in  the  least  surprising, 
when  it  is  recalled  that  in  the  cone,  sedimentation  is  quiescent 
and  theoretic  retention  is  100  per  cent,  efficient — -while  in  the 
tank  there  is  always  some  movement  in  the  settling  sewage  and 
the  depth  is  several  times  greater  than  in  the  cone,  and  it  follows 
that  particles  reach  the  bottom  of  the  cone  considerably  quicker 
than  they  do  the  bottom  of  the  tank. 

In  the  studies  made  on  the  Brooklyn  sewage,  experiments  were 
undertaken  by  Mr.  W.  T.  Carpenter  to  ascertain  some  of  the 
phenomena  attending  the  sedimentation  of  sewage  solids.  These 
studies  were  too  elaborate  to  be  given  space  in  this  paper,  but 
as  one  set  of  experiments  illustrates  the  effect  of  depth  on  sedi- 
mentation, and  fineness,  or  colloidal  condition  of  suspensa,  suffi- 
cient of  it  will  be  given  to  bring  out  this  point. 

The  apparatus  used  consisted  of  2-inch  diameter  galvanized 
iron  pipe  terminating  in  a  coupling  into  which  was  cemented  the 
lower  part  of  an  Imhoff  cone,  thus  giving  a  chamber  in  which 
settling  matter  was  visible  and  could  be  measured  volumetrically. 
The  apparatus  afforded  a  sedimentation  depth  of  74  inches.  The 
capacity  was  that  of  a  measured  sample  bottle,  which  was  filled 
with  the  sewage  or  tank  effluent  to  be  tested.  The  contents  of 
the  bottle  were  discharged  into  the  apparatus  as  quickly  as  pos- 
sible. The  volume  of  sediment  forming  at  the  apex  was  read  at 
intervals  of  15  minutes,  30  minutes,  and  at  1,  2,  3,  4,  5,  and  6 
hours. 

A  large  number  of  such  tests  were  made  on  crude  sewage,  and 
also  on  Imhoff  tank  effluent,  using  that  from  tank  3. 

The  figures  given  are  averages  of  all  the  tests.  For  compari- 
son, figures  showing  rate  of  deposit  in  the  ordinary  Imhoff  cone 
on  the  same  sewage  and  effluents  are  also  given. 


Sewage  Treatment  Experiments 


25 


TABLE  IX 

Volume  of  Deposit  and  Time  of  Settling 

Observed  in  Im/iofi  and  Special  Cones 
Quantities  in  c.c.  per  liter 


Time 

Imhoff  Cone 

Special  74- 

in.  Cone 

Sewage 

Effluent 

Sewage 

Effluent 

Minutes 

S 

0.63 

0.0 

10 

1.11 

.12 

IS 

1.19 

.20 

0.52 

0.07 

30 

1.36 

.29 

.90 

.10 

45 

1.48 

.36 

.13 

Hours 

1 

1.49 

.40 

1.33 

.15 

2 

1.90 

.47 

1.69 

.20 

3 

2.30 

.50 

1.89 

.25 

4 

1.97 

.28 

5 

2.02 

.32 

6 

2.03 

.a 

Sewage  and  effluent  tested* 

Contents 

of 

Sewage 

Effluent 

%  Removed 

Total    solids 

162  p. p.m. 

97  p. p.m. 

40 

Settling 

solids 

92  p. p.m. 

23  p. p.m. 

66 

Colloidal 

solids 

93  p. p.m. 

74  p. p.m. 

20 

*  Effluent  is  from  Imhoff  Tank  3. 

%  of  removal  given  was  effected  in   the  Imhoff  tank. 


Experimental  Data  From  the  Imhoff  Tanks  and  Settling  Tanks 
(Dortmund  Type)  With  Short  Retention  Periods. 

The  station  was  provided  with  three  Imhoff  tanks,  so  designed 
that  they  differed  in  depth  and  cubic  capacity  only.  Each  tank 
was  a  wooden  cylinder,  15  ft.  in  diameter.  They  were  placed 
along  the  northerly  side  of  the  plant,  extending  east  from  the 
quieting  tank,  from  which  each  received  sewage  thru  an  inde- 
pendent flume,  the  sewage  entering  the  flume  thru  an  adjustable 
measuring  orifice.  The  plant  was  also  provided  with  four  plain 
settling  tanks  (Dortmund  type),  which  received  sewage  from 
the  quieting  tank  in  the  same  manner. 

The  principal  dimensions  of  these  tanks  are  shown  by  means 
of  Table  X,  and  the  accompanying  sketches. 

For  the  purposes  of  this  paper  the  three  Imhoff  tanks  and 
settling  tank  3,  will  be  considered  in  one  group,  as,  during  the 
period  of  the  tests  described  on  the  Imhoff  tanks,  settling  tank  3 
was  operated  on  the  same  sewage  in  connection  with  a  separate 
digestion  tank,  which  daily  received  the  solids  settled  out.  The 
two  tanks  acted  the  part  of  an  Imhoff  tank  with  the  two  stories 
separated. 

The  direction  of  flow  thru  the  Imhoff  tanks  was  from  north  to 
south.  The  tanks  were  so  connected  with  the  other  units  of  the 
experimental  plant,  that  Imhoff  effluent  could  be  obtained  by  grav- 
ity flow  for  all.  The  three  tanks  were  similar  in  design.  Inlet 
and  outlet  weirs  were  full  width  of  the  flowing-thru  chamber, 
and  were  of  the  same  design  in  each  case,  as  were  also  the  hopper 


26 


Brooklyn,  N.  Y. 


Fig.  7.     View  of  Experimental  Tanks  from  the  North. 

1.  Quieting  Tank. 

2.  Imhoff  Tank  No.  1. 

3.  Imhoff  Tank  No.  2. 

4.  Imhoff  Tank  No.  3. 

5.  Sprinkling  Filter  Beds. 

Capacity  of  Plant — 1,200,000  gallons  per  day  of  sewage  was  used  in  the 
various  experimental  processes. 

Man  is  seen  taking  samples  on  top  deck  of  Imhoff  Tank  No.  3,  just 
ui\der  Fig.  4. 

The  Imhoff  Tanks  are  15  feet  in  diameter  inside. 


bottoms,  the  8-inch-diameter  sludge  outlet,  the  settling-chamber 
floors,  slopes  and  slots.  The  tanks  differed  only  in  the  matter  of 
depth.     The  water  line  in  each  was  at  elevation  31.17. 

The  slots  for  the  passage  of  settled  matters  from  the  slopes 
into  the  digesting  chamber  were  so  •  designed  that  the  plane  of 
each  slope  was  carried  without  obstruction  directly  thru  into 
the  digesting  chamber,  on  each  side.  The  slopes  did  not  pass  the 
one  under  the  other,  as  is  often  observed  in  American  practice. 
The  latter  design  was  avoided,  as  with  it  the  settlings  on  one 
slope  must  turn  over  upon  the  other  slope  in  order  to  pass  thru 
into  the  digesting  chamber,  thus  making  obstruction  probable  and 
calling  for  frequent  cleaning  of  slopes  and  slots.  See  Fig.  9,  at 
1  and  3.  The  design  adopted  to  avoid  this  (Fig.  9,  at  2)  was 
to  guard  the  slots,  or  openings  from  below,  from  rising  gases  by 
means  of  an  A-shaped  shield  or  baffle  which  afforded  a  slot  on 
each  side.  The  upper  surfaces  of  the  A-shaped  shield  were  ex- 
actly in  the  same  plane  as  the  slopes  of  the  flowing-thru  chamber, 
so  that  when  sediment  started  to  slide,  it  found  an  opening  di- 
rectly in  its  path  thru  which  it  could  pass  without  stopping  or 
turning  in  its  course.     This  is  believed  to  be  an  important  prac- 


Sewage  Treatment  Experiments 


27 


tical  point  in  the  design  of  these  tanks.  It  was  so  successful  that 
slopes,  inclined  only  about  42  degrees  from  level,  did  not  at  any 
time  become  clogged  in  five  years'  service,  and  never  needed  to 
be  squeegeed  down.  The  width  of  each  slot  on  the  plane  of  the 
slope  was  6  inches.  The  inclination  of  slope  above  mentioned 
was  put  in  as  the  least  slope  that  might  be  expected  to  work  with 
success.  Arrangements  had  been  made  to  increase  it  if  neces- 
sary, but  no  such  change  was  made. 


SECTIONS  OF  IMHOFF  TANKS 
IMHOFF  TANK  N?  I  IMHOFF  TANK  N?  2 

fwvf 


SLUW.E  PSCH&AbE 


IMHOFF  TANK  N? 3 

■=» 


o    a    ti 


"3  H  5 


Fie.  8. 


The  bottom  of  each  digesting  chamber  was  formed  inside  of 
the  cylindrical  tank  in  the  shape  of  an  inverted  truncated  hexa- 
gonal pyramid,  made  in  two  sections,  the  upper  overlapping  the 
lower.  A  perforated  lead  pipe  1%  inches  in  diameter  was  placed 
entirely  around  the  overhanging  edge  of  the  upper  section  of 
pyramid,  and  connected  with  the  city  water  supply,  for  use  in 
starting  sludge,  etc.  The  water  was  controlled  by  a  gate  valve. 
The  use  of  this  pipe  never  was  called  for  in  the  operation  of  the 
tanks. 

Scum  boards  18^  inches  deep  were  placed  at  inlet  and  outlet 
weirs,  one  foot  from  each  weir,  and  were  used  thruout  the  ex- 
periments. 

Baffles  were  not  provided  at  first,  but  the  necessity  for  them 
was  demonstrated  as  the  experiments  progressed,  and  they  were 
then  put  in,  after  considerable  study  as  to  their  position,  shape 
and  depth. 


Fig.  9. 

1.  Type  of  Slot  Frequently  Used  in  American   Practice. 

2.  Type   of  Slot  Used  on  all   the  Brooklyn   Imhoff  Tanks. 

3.  Type  Sometimes  Used. 


Sewage  Treatment  Experiments 


29 


PLAN  OF  IMHOFF  TANKS 
IMHOFF  TANK  N9  I  IMHOFF  TANK  N<?£  IMHOFF  TANK  N?  3 


FLUMES  fTOM  QUIETtS6   TftHK 


,  EFFLUENT  FROM   IMHOFF    TANKS    TO  i    &al       TRICKLW6 

yCX __^_ __ {J = 


nus&um  Wfftct  tat  ■ 

01   e  3  4  s  ;. 

FEET 


Co? 

Fig.  10. 


HasnmMRcei 


Fig.  11. 


The  gas  vents,  being  segments  of  the  15  ft.  diameter  circle, 
cut  off  by  the  walls  of  the  flowing-thru  chambers,  were  2  ft.  wide 
at  their  greatest  width.  They  were  at  all  times  ample  for  the 
purpose. 

The  tanks  were  completed  and  put  in  service  October  1,  1913, 
and  a  few  days  later,  at  the  suggestion  of  Dr.  Rudolph  Hering, 
ripe  sludge  was  obtained  from  the  tanks  of  the  Pennypack  Creek 


30  Brooklyn,  N.  Y. 

sewage  disposal  plant  near  Philadelphia,  and  with  this  each  tank 
was  seeded.  The  ripening  period  extended  thru  the  severe  win- 
ter of  1913-1914  and  was  not  completed  in  either  of  the  tanks 
until  May  21,  1914,  when  No.  2  and  No.  3  gave  ripe  sludge,  but 
No.  1  did  not  ripen  until  June  11.  The  tanks,  however,  all  con- 
tinued to  improve  in  operation  during  the  year  1914.  Early  in 
the  spring  there  was  considerable  smell  from  all  of  them,  but 
this  disappeared  during  the  summer,  and  did  not  return  until  the 
following  spring.  Improvement  continued  thruout  the  entire 
period  of  operation,  and  the  tanks  were  doing  their  best  when 
finally  shut  down  in  1918.  At  no  time  during  the  experiments 
was  there  any  evidence  of  sludge  accumulating  on  the  slopes  of 
the  settling  chambers.  As  stated  elsewhere,  the  slopes  inclined 
only  41  degrees  43  minutes  from  level.  It  had  been  intended  to 
try  a  number  of  different  angles  of  slope,  beginning  with  the  least 
which  it  was  convenient  to  place,  with  the  expectation  that  clean- 
ing would  be  necessary  with  slopes  inclined  from  level  less  than 
50  degrees,  but  there  seemed  no  reason  to  increase  the  inclination 
after  installing  the  slopes.  The  surfaces  of  the  slopes  were  of 
planed  boards,  with  the  grain  running  with  the  slope.  It  is  quite 
possible  that  this  surface  is  less  subject  to  affording  points  of  ad- 
hesion for  settling  matters  than  concrete  would  be,  but  as  it  was 
considered  perfectly  feasible  to  use  an  inclined  floor  of  planed 
boards  for  the  slopes  in  a  full  size  plant,  it  was  decided  not  to  go 
further  in  this  study,  but  to  adopt  an  angle  of  inclination  in  future 
tanks  as  much  greater  than  42  degrees  as  the  other  conditions 
would  permit.  The  double  slot  placed  at  the  lowest  point,  i.  e. 
the  intersection  of  the  planes  of  each  side,  without  permitting  the 
crest  of  the  baffle  to  emerge  into  the  settling  tank  and  form  an 
obstruction  to  sliding  matters, — was  of  much  importance  in  keep- 
ing the  slopes  clean.  The  settlings  do  not  appear  to  go  thru  the 
slot  at  the  foot  of  the  slope  down  which  they  slide,  unless  they 
are  heavy.  The  light-weight  materials  and  much  of  the  heavy, 
appear  to  jump  over  this  space  and  go  thru  the  other  slot,  which 
does  not  require  a  change  in  their  direction  of  movement  (see 
Fig.  9).  It  appears  from  the  work  of  the  Brooklyn  tanks  that 
the  design  of  slots  is  far  more  important  in  its  relation  to  tank 
operation  than  has  been  generally  recognized  in  practice. 

The  phenomena  which  accompany  foaming  and  frothing  in 
the  Imhoff  tanks  are  so  various  that  it  must  be  admitted  that 
many  explanations  are  possible.  Tanks  will  foam  when  no 
sludge  is  present,  and  also  when  filled  with  sludge — and  when 
much  or  little  is  present.  The  suspended  matters  are  so  light  in 
specific  gravity  that  any  cause  which  gives  rise  to  a  rapid  gas 
formation  may  result  in  foaming,  even  changes  of  barometric 
pressure  in  the  atmosphere  may  cause  it.  The  presence  of  grease 
in  the  tank  and  an  acid  reaction,  seems  to  be  one  of  the  most 
persistent   conditions    noted    when    foaming   takes    places.      But 


Sewage  Treatment  Experiments  31 

foaming  takes  place  when  the  condition  is  not  acid  and  there  is 
but  little  grease  present,  and  when  explanation  seems  impossible. 
At  times  during  the  experiments,  and  without  any  apparent  rea- 
son, the  entire  body  of  sludge  in  the  digestion  chambers  of  the 
different  tanks  would  rise  to  the  top,  usually  without  foaming  or 
frothing,  and  remain  for  a  day  or  two,  then  slowly  settle.  On 
such  occasions,  if  foaming  did  not  occur  gas  discharge  from  the 
gas  vents  was  at  least  more  than  usually  active  and  large  eructa- 
tions of  foul  gas  were  noted.  At  such  times  there  was'  considera- 
ble odor  present,  and  when  foaming  occurred  there  was  a  strong 
and  very  persistent  odor.  Churning  the  scum  with  a  hoe  in  the 
gas  vents,  or  breaking  it  up  with  a  paddle,  usually  sufficed  to 
release  the  entrained  gas  and  permit  it  to  settle.  A  jet  from  a 
hose  is  probably  the  most  effective  method  of  doing  this.  At  no 
time  was  the  foaming  so  serious  that  it  could  not  be  controlled, 
but  while  it  lasted  the  tank  gave  forth  bad  smells,  and  its  effi- 
ciency as  a  remover  of  suspended  matter  decreased  more  than 
half.  The  settling  chambers  were  at  no  time  involved  in  foaming 
and  very  seldom  showed  any  scum. 

The  regrettable  thing  about  this  bad  habit  of  the  Imhoff  tank 
is  that  even  if  it  can  be  controlled  it  causes  bad  odors,  which 
makes  it  a  very  uncertain  risk,  and  a  bad  neighbor,  and  suggests 
that  such  tanks  should  not  be  placed  near  enough  to  habitations 
to  give  rise  to  damage  claims  or  cause  complaint.  If  it  is  neces- 
sary to  put  such  a  tank  near  inhabited  structures,  where  com- 
plaint is  at  all  probable,  it  should  be  covered  with  a  building  that 
would  keep  in  the  odors,  when  these  occur.  It  is  a  simple  matter 
to  provide  a  building  which  will  insure  a  neighborhood  against 
nuisance  from  odors  and  at  the  same  time  permit  the  use  of  this 
tank,  which  is  otherwise  such  an  excellent  apparatus  for  sewage 
treatment. 

The  settling  tanks  (Dortmund  type)  never  showed  the  least 
inclination  to  foam,  and  never  gave  forth  odors  that  could  be 
recognized  as  coming  from  sewage  a  few  feet  away.  This  may 
have  been  because  they  were  not  intended  for,  or  used  as  digest- 
ing or  septic  tanks,  the  settling  matters  and  sludge  being  removed 
daily  for  other  treatment. 

It  may  be  added  that  the  Imhoff  tanks  very  seldom  caused 
odors,  and  at  times  when  they  did,  if  it  was  not  due  to  foaming, 
it  was  probably  due  to  a  septic  condition  of  the  sewage  entering 
them.  At  one  time  during  the  experiments  all  of  the  tanks  caused 
odors,  and  it  was  discovered  that  the  main  sewer  had  become 
very  foul  from  deposits.  After  the  sewer  was  cleaned  out  these 
odors  ceased,  and  did  not  occur  until  the  sewer  again  required 
cleaning.  When  this  was  done,  odors  again  ceased,  at  least 
suggesting  the  connection  between  bad  smells  from  a  disposal 
plant,  and  neglect  in  keeping  sewers  clean. 

The  idea  that  sewers   and   disposal   plants   will    run   without 


32  Brooklyn,  N.  Y. 

causing  trouble  if  left  to  take  care  of  themselves,  is  erroneous 
as  we  all  know,  but  it  is  not  sufficiently  understood  that  troubles 
with  the  disposal  plant  are  frequently  caused  by  neglect  in  clean- 
ing out  the  sewers. 

For  some  reason  not  understood  by  the  writer,  the  Imhoff 
tank,  while  being  installed  more  and  more  extensively  by  engi- 
neers, has  fallen  into  disrepute  in  public  opinion.  In  rather  ex- 
tensive travels  around  this  country  during  the  last  few  years, 
the  writer  has  visited  many  such  tanks,  and  plans  of  new  installa- 
tions are  constantly  appearing  in  the  technical  papers',  yet,  on 
visiting  various  plants  in  operation,  many  complaints  were  heard, 
and  the  general  consensus  of  opinion  was  decidedly  unfavorable. 
What  is  the  reason  for  this  attitude  on  the  part  of  those  who  are 
maintaining  and  operating  these  tanks  ?  There  is  nothing  worse 
than  to  give  a  dog  a  bad  name,  and  many  a  good  dog  has  been 
condemned  for  this  reason. 

The  Imhoff  tank  has  been  closely  studied  by  the  writer,  and 
while  his  judgment  may  be  prejudiced  in  its  favor,  and  he  may  be 
told  that  his  explanations  of  its  failures  are  nothing  more  than 
apologies,  yet  he  feels  that  they  are  entitled  to  a  hearing  at  least. 
His  first  studies  of  the  tank  were  received  from  Dr.  Imhoff, 
himself,  nearly  ten  years  ago,  and  he  had  the  opportunity  of 
visiting  with  Dr.  Imhoff  all  of  the  larger  and  some  of  the  smaller 
installations  in  the  Emscher  district  in  1912,  and  hearing  at  first 
hand  the  principles  upon  which  the  designs  were  based,  and  of  the 
various  investigations  which  had  been  made  on  various  points 
concerned  both  in  the  design  and  in  the  operation  of  these  tanks. 
In  1913  Dr.  Imhoff  spent  some  of  his  valuable  time  in  the  writer's 
office  in  Brooklyn,  discussing  the  design  of  the  three  tanks  de- 
scribed in  this  paper,  he  having  permitted  their  use  without 
charge  for  experimental  purposes.  Many  of  the  features  of  the 
tanks  were  made  to  conform  especially  with  his  views  and  in- 
structions, so  that  the  design  in  reality  was  his  and  not  the 
writer's — and  to  him  alone  is  due  the  credit  for  their  success. 
The  writer  has,  since  1912,  visited  many  American  installations 
of  the  Imhoff  tank,  and  while  he  must  admit  some  troubles  in 
operation,  and  smelling  some  odors  not  very  bad  to  a  sewage 
expert,  but  killing  to  neighboring  residents  who  had  the  possi- 
bility of  damages  in  prospect,  he  feels  that  the  public  has  been 
unjust  in  its  generally  unfavorable  attitude.  There  were,  how- 
ever, not  a  few  instances  of  troubles,  for  some  of  which  the 
designers,  and  not  Dr.  Imhoff,  were  to  blame,  and  others  which 
were  due  to  mismanagement,  and  still  others  to  gross  overload- 
ing, and  even  to  entire  neglect.  The  tank  is  admitted  to  be  deli- 
cate to  operate,  and  requires  constant  care  and  good  manage- 
ment, but  this  is  true  of  disposal  plants  generally. 

It  was  assumed  by  the  designers  of  the  early  American  Imhoff 
tanks,  that  since  American  sewage  is  weaker  than  German,  the 


Sewage  Treatment  Experiments 


33 


digestion  chamber  of  the  tank  might  be  made  smaller.  But  the 
truth  probably  in  most  cases  is  that  American  sewage  as  a  rule 
forms  a  sludge  with  a  larger  per  cent,  of  water  than  is  the  case 
abroad,  and  that  its  volume  is  actually  greater  in  consequence,  and 
requires  more  space  for  digestion. 

Whatever  the  various  causes  of  foaming  may  be,  it  appears 
certain  that  sludge  arising  above  the  slot-line  will  cause  it,  as 
well  as  turn  the  entire  tank  above  and  below  into  an  ordinary 
septic  tank.  And  whenever  this  condition  does  occur  an  active 
nuisance  from  smells  is  inevitable.  In  spite  of  all  that  can  be 
said  against  the  tank,  it  must  be  allowed  that  its  advantages  are 
such  that  a  certain  amount  of  the  risk  of  nuisance  may  be  justi- 
fied. But  one  should  not  hide  the  truth  that  such  a  risk  exists, 
and  every  tank  which  is  not  carefully  operated  will  probably 
cause  local  trouble. 

The  design  of  these  tanks  calls  for  the  highest  knowledge  of 
sewage  treatment,  and  of  the  peculiar  qualities  of  sewage  solids, 
grease,  etc.,  and  should  never  be  undertaken  without  the  most 
careful  study.  Not  a  few  engineers  who  have  ventured  into  this 
field  look  back  with  sorrow  on  their  first  tanks ;  and  this  has  had 
much  to  do  with  public  feeling  regarding  the  Imhoff  tank. 

TABLE  X 
Tank  Data,  Imhoff  Tanks 

fmhoff  Tanks 
1  2  3 

Elev.  water  line,   ft.: • 31.17  31.17  31.17 

Elev.  bottom  of  tank,  ft 0.79  9.29  17.50 

Elev.   slope  intersection,   ft 17.21  21.12  24.17 

Diameter   of    tank,    ft 15.00  15.00  15.00 

Depth,    total,    ft 30.38  21.88  13.67 

Width  of  settling  chamber,   ft 10.67  10.67  10.67 

Depth  of  settling  chamber  to  slots,  ft 13.97  10.05  7.00 

Depth  of  settling  chamber  to  slopes,  ft 9.22  5.30  2.42 

Slopes  inclined   from  level,  degs 41°  43'  41©  43'  410  43' 

Depth  of  scum  boards,  ft 1.54  1.54  1.54 

Depth  of  baffles,   ft 9.50  7.79  5.54 

Capacity  below  w.  1.,   cu.   ft 1,610.00  1,113.00  557.00 

Capacity  below  w.   1.,  gals 12,050.00  8,325.00  4,166.00 

Capacity  1   hr.   retention,  gals 290,000.00  200,000.00  100,000.00 

Capacity  2   hr.   retention,  gals 145,000.00  100,000.00  50,000.00 

Digesting  chamber  diameter,   ft 15.00  15.00  15.00 

Digesting  chamber  depth,  ft 16.42  11.83  6.67 

Capacity  below  slot-line,  cu.   ft 2,128.00  1,317.00  469.00 


TABLE  XI 
Tank  Data,  Settling,  Digestion  and  Humus  Tanks 

Settling  Digestion 
Tanks  1-4  Tank 

Elev.  water  line,   ft 7.80  -|-6.50 

Elev.  top  of  hopper  bottom,  ft 3.80  ■ — 4.50 

Elev.  bottom  hopper  bottom,   ft 0.20  — 7.00 

Depth  below  w.  1.,  ft 7.60  13.50 

Size  water  surface,  ft 8x8  D.5.00 

Capacity,    cu.    ft 285.00  265.00 

Capacity,    gals 2,132.00  1,988.00 

Capacity  1   hr.  retention,  gals 51,168.00  

Capacity  2  hr.   retention,  gals 25,584.00  


Humus 
Tanks 


6.02 


—3.90 

9.92 

5.6x5.6 

127.00 

950.00 
22,800.00 
11,400.00 


34  Brooklyn,  N.  Y. 

Theoretic  and  Observed  Retention 

The  term  "theoretic  retention"  in  this  study  means  the  time 
required  to  fill  a  tank  at  the  rate  of  flow  under  consideration. 
It  is  well  known  that  the  true  retention  is  always  less  than  the 
theoretic,  and  to  what  extent  the  tanks  at  the  station  differed  from 
the  theoretic  was  of  much  interest  in  the  interpretation  of  phe- 
nomena. Observation  led  to  the  conclusion  in  advance  of  study, 
that  the  difference  between  the  theoretic  and  the  actual  must  be 
considerable. 

Different  means  of  determining  this  point  were  tried,  and  in 
fact  none  proved  entirely  satisfactory.  That  which  was  consid- 
ered the  best  was  determination  by  means  of  dyes.  Ammonium 
chloride  and  sodium  chloride  were  previously  tried,  but  were 
rejected  because  of  the  high  specific  gravity  of  their  solutions, 
which  caused  them  to  fall  to  the  bottom  and  pass  out  very  slowly, 
giving  too  long  a  period. 

Fuchsin  dye,  by  reason  of  its  great  strength  and  the  ease  of 
comparing  its  solutions  with  standards,  gave  fair  results.  The 
dye  was  applied  at  the  entrance  end  of  the  tank,  and  samples  were 
taken  from  the  effluent  at  short  intervals  until  it  had  disappeared. 
The  intensities  of  color  expressed  in  per  cent,  of  maximum  were 
plotted  as  ordinates  with  time  intervals  from  the  application  as 
abscissae,  and  a  curve  adjusted  to  them.  The  area  of  this  curve 
should  represent  the  total  quantity  of  dye  used. 
1  The  portion  of  the  area  to  the  left  of  any  ordinate  line  should 
represent  the  dye  which  has  passed  out,  and  that  to  the  right,  the 
dye  still  retained  in  the  tank. 

The  abscissa  of  that  ordinate  that  bisects  the  area  should, 
therefore,  be  the  time  of  retention.  This  represents  the  time  of 
the  passage  of  the  sewage,  and  should  be  the  actual  retention 
period.  There  are,  however,  a  number  of  difficulties  with  the 
method.  To  recognize  the  actual  first  appearance  of  color  in  the 
effluent  is  not  easy,  nor  is  it  easy  to  recognize  the  last  trace,  and 
the  extent  to  which  fine  colloidal  matter  in  the  sewage  and  set- 
tling solids  may  carry  down  the  color  is  also  an  unknown  factor. 
The  results  of  the  tests,  however,  are  of  much  interest  and  give 
valuable  comparative  information.  The  accuracy  of  the  tests  and 
the  conclusions  founded  on  them  are  questionable,  and  the  results 
should  not  be  taken  as  demonstrations  of  the  actual,  or  anything 
near  the  actually  true  retention,  under  average  conditions  of 
operation. 

A  difficulty  that  appears  to  be  fundamental  is,  that  such  ob- 
servations hold  good  only  for  the  retention  period  from  which 
they  are  obtained.  Thus  a  tank  may  show  the  true  retention  to 
be  80  per  cent,  of  the  theoretic  for  a  3-hour  retention  period, 
while  it  may  also  be  found  only  40  per  cent,  of  the  theoretic 
for  a  2-hour  retention  period.     In  other  words,  the  tank  is  not 


Sewage  Treatment  Experiments 


35 


standardized  by  this  method  for  all  periods,  and  cannot  be,  for  its 
efficiency  is»  different  with  every  rate  of  retention.  Each  tank, 
however,  has  its  best  retention  period. 

As  the  2-hour  period  appeared  to  be  the  best  suited  for  tank 
operation  at  this  location,  most  of  the  experimental  work  was 
done  with  this  period. 

The  following  statement  shows  the  result  of  the  Fuchsin  dye 
test  on  the  different  tanks  : 

TABLE  XII 

Imhoff  Theoretic  Sewage  Observed  Loss  of  Suspensa 

Tank  Retention  Per  Day  Retention  Retention  Removed 

No.                               hours  gals.  hours  %  % 

1 2  145,000  1.00  50  61 

2 2  100,000  1.00  SO  56 

3 1  100,000  0.33  68  51 

Settling 
Tank  No. 

3 2 50,000 L77 10 77 

As  observed  by  use  of  Fuchsin  dye. 
Observations  in  June-July,   1915. 
Tanks  all  working  poorly  except  settling  tank  3. 
Suspensa  removed  determined  by  Imhoff  cone. 

Effect  of  Baffling  on  Imhoff  Tanks 

As  originally  constructed  the  tanks  were  not  provided  with 
baffles,  as  it  was  intended  to  determine  by  observation  whether 
these  were  necessary,  and  if  so,  the  proper  depth  and  location 
for  them. 

When  all  the  tanks  were  operated  on  a  2-hour  theoretic  reten- 
tion, during  a  3-months  period,  their  comparative  efficiency  in 
the  removal  of  settling  matter  and  suspended  solids  was  noted, 
revealing  marked  differences  between  them.  The  data  led  to  the 
conclusion  that  the  flow  in  all  three  tanks  reached  down  only  to  a 
limited  depth,  regardless  of  the  depth  of  the  settling  chamber. 
The  dimensions  of  these  chambers  are  shown  in  Table  X.  The 
length  and  breadth  of  flow  is  equal  in  each  settling  chamber,  but 
the  depth  and  cubic  capacity  of  each  is  different  from  the  others. 
To  agree  with  theoretic  retention,  with  the  same  quantity  of 
sewage  flow  passed  into  each  tank,  the  retention  period  should 
agree  directly  with  the  cubic  capacity.  To  test  this  theory  the 
three  tanks  were  each  put  in  operation  at  the  rate  of  75,000 
gallons  per  day,  giving  the  following  theoretic  retention  to  each, 
with  the  result  shown  : 


Imhoff 
Tank  No. 

Theoretic 
Retention 

Sewage 
Per  Day 

Suspensa 
Removal 

1 

2 

3 

hours 

3.8 

2.7 

1.3 

gals. 

75,000 
75,000 
75,000 

% 

29 
30 
15 

Rate  continued  April   1  to  June  30,  1914. 

The  average  results  revealed  nearly  equal  removals  in  tanks 
1  and  2,  and  an  apparent  loss  in  tank  3. 


36 


Brooklyn,  N.  Y. 


After  considerable  study  on  small  models  and  a  good  deal  of 
discussion  on  the  hydraulics  of  the  tanks,  baffles  were  placed  6 
ft.  from  the  entrance  weir  of  the  chambers,  which,  being  15  ft. 
long,  left  9  ft.  from  the  baffle  to  the  outlet  weir.  This  was  found 
to  be  the  best  location,  at  least  for  these  tanks,  and  the  baffles 
remained  in  this  position  until  1917,  when  some  further  experi- 
ments were  made  with  the  baffles  in  various  situations.  For 
depth  of  baffles  see  Table  X. 

With  the  baffles  thus  located  the  following  data  show  the  re- 
sults obtained. 


Imhoff 
Tank  No. 

Theoretic 
Retention 

Sewage 
Per  Day 

Suspensa 
Removed 

1 

2 

3 

hours 

2 

2 

2 

gals. 
145,000 
100,000 

50,000 

% 
34 
35 
44 

July  and  August,  1914. 

The  demonstration  was  quite  convincing  that  at  least  for 
these  tanks  baffles  were  required.  And  it  will  be  noted  that  the 
improvement  was  greatest  in  the  shallowest  of  the  three  tanks, 
which  was  rather  an  unexpected  result. 

Floating  Scum  on  Imhoff  Tanks 

Floating  scum  was  but  seldom  present  in  the  settling  cham- 
bers, and  when  present  usually  consisted  mainly  of  grease  which 
separated  out  from  the  sewage  matters.  The  scum  boards  were 
quite  efficient  in  preventing  floating  matters.  Such  scum  and 
other  materials  as  collected  behind  the  scum  boards  were  paddled 
from  time  to  time,  and  settled  without  causing  much  trouble. 
When  necessary,  materials  that  did  not  settle  were  thrown  into 
the  gas  vents. 

Floating  scum  in  the  gas  vents,  taking  the  period  from  Jan- 
uary, 1915,  to  the  end  of  April  as  representative,  was  made  up 
as  follows : 

TABLE  XIII 

Quantities  Given  in  Per  Cent,  of  Total  Constituents 

Imhoff  Tank 
1  2  3 

% 

Moisture    80.2 

Solids     19.8 

Mineral 26.0 

Organic    74.0 

Fats     26.6 

Average  depth  of   scum,   inches 3.75 


% 

% 

81.5 

84.6 

18.5 

15.4 

27.7 

30.5 

72.3 

69.5 

25.4 

11.2 

1.50 

oy2 

Effect  of  Imhoff  Tanks  on  Bacterial  Content  of  Sewage 

The  effect  of  passing  sewage  thru  tanks  on  the  bacterial  con- 
tent of  the  sewage  is  a  doubtful  matter,  as  the  results  are  so 
various  and  differ  so  much  with  the  different  methods  employed. 
The  determination  of  this  for  the  Brooklyn  sewage  was  rather 


Sewage  Treatment  Experiments 


37 


more  interesting  than  important.  It  was  assumed  that  should 
disinfection  prove  necessary  a  trickling  filter  plant  would  be  in- 
stalled, and  the  question  of  bacterial  removal  would  arise  only 
in  connection  with  an  oxidized  effluent.  Moreover,  the  writer 
believes  that  much  discrimination  is  called  for  in  selecting  and 
interpreting  the  results  of  a  purely  biological  study,  and  the  scope 
of  the  experimental  work  did  not  require  that  this  be  included  as 
of  primary  importance,  but  a  means  of  interpreting  other  work 
and  checking  results.  In  the  warmer  months  of  the  year  with 
septic  sewage  and  much  daily  departure  from  average  biological 
conditions,  the  results  would  be  masked  by  these  conditions  and 
of  no  great  value,  and  might  mislead.  In  the  cold  months  no 
valuable  results  bearing  on  the  local  problem  were  to  be  expected. 
It  therefore  appeared  that  the  results  obtained  in  the  spring,  espe- 
cially March  and  April,  were  about  the  best  for  this  study,  and 
the  city  was  fortunate  in  having  some  tests  made  by  experts  in 
this  line  of  work  at  the  very  time  when  the  results  would  be  of  the 
most  interest,  and  not  marked  by  the  many  misleading  factors 
existing  in  summer  and  winter. 

The  tests  referred  to  were  made  by  Messrs.  Lederle  &  Provost, 
at  the  Lederle  laboratories  in  New  York,  and  at  the  experiment 
station  laboratory,  in  March  and  April,  1915,  in  connection  with 
a  study,  made  with  the  city's  consent,  for  a  client  who  had  re- 
tained these  experts  to  experiment  with  and  report  on  ozone 
disinfection  of  sewage  and  effluents. 

The  sewage  tested  was  taken  from  the  ordinary  crude  sewage, 
and  the  Imhoff  tank  effluent  represents  tank  2  only.  The  reten- 
tion period  was  2  hours. 

TABLE  XIV 

Average  Bacterial  Counts,  and  Chemical  Constituents  With  Per- 
centages of  Reduction 

March  24  to  April   13,   1915,  Inclusive — Parts  Per  Million 


Crude 
Sewage 

Dissolved    oxygen    consumed 148. 

Total   suspended  matter 256. 

Mineral,    in    suspensa 72. 

Organic,    in    suspensa. 184. 

Total  solids  in  sewage 739. 

Total  minerals  in  sewage 361. 

Total  organics  in  sewage 378. 

Nitrates     , 0.38 

Oxygen    consumed 91 

Tank  retention  (theoretic),  2  hours. 

Bacterial  Counts  Per  C.  C. 

Crude 
Sewage 

Agar,  24  hrs.  at  37oC 250,000 

Gelatin,  48  hrs.  at  20 oC 600,000 

Bacteria  of  B.  Coli  type 42,000 

— ■  Indicates  reduction. 
-I-  Indicates  increase. 


Effluent  from  % 

Imhoff  Tk.  2      Reduction 


89 

39.9 

179. 

30.1 

42. 

41.7 

137 

25.5 

618 

16.4 

326 

9.4 

292 

22.8 

0.25 

34.2 

72 

20.8 

Imhoff 
Effluent 


%  Changed 


214,000 
988,000 
107,000 


—14.4 

-1-64.7 
-1-154.3 


38 


Brooklyn,  N.  Y. 


It  should  be  noted  that  the  sewage  during  the  period  of  these 
biological  tests  was  considerably  stronger  than  the  averages 
for  the  two  months,  parts  of  which  fell  into  the  test  period.  This 
was  probably  caused  by  the  movement  down  the  mains  of  the 
sewage  system  of  the  deposits  that  had  been  formed  during  the 
winter,  brought  about  by  melting  snows  and  rains.  On  account 
of  conditions  caused  by  a  heavy  rain  fall,  the  sewage  and  tank 
effluent  results  for  March  26  were  excluded  from  the  above  table. 
As  these  results  are  of  interest  as  showing  what  storm  conditions 
can  do  at  this  plant,  they  are  given  below : 

TABLE  XV 

Bacterial  Count  and  Chemical  Constituents  of  Sewage 

March   26,   1915— Parts   Per  Million 

Crude  Effluent  from  % 

Sewage  Imhoff  Tk.  2      Reduction 

Dissolved  oxygen  consumed 143.5  105.  27. 

Total   suspended   matter 236.  172.  27. 

Minerals   in    suspensa 74.  26.  70. 

Organics    in    suspensa _ 162.  146.  10. 

Total  solids  in   sewage 822.  606.  26. 

Total  minerals  in  sewage 380.  352.  7. 

Total  organics  in  sewage 442.  254.  42. 

Nitrates     0.42                     0.14  41. 

Oxygen    consumed 77.5  67  13. 

Tank  retention  (theoretic),  2  hours. 

Bacterial  Counts  Per  C.  C. 

^  

Crude  Imhoff 

Sewage  Effluent  %  Changed 

Agar,  24  hrs.  at  37  C 20,000,000  4,752,000  —76. 

Bacteria  of  B.   Coli  type 1,000,000  258,000  —74. 

—  Indicates  reduction. 

It  is  obvious  from  the  above  that  the  storm-sewage  problem 
in  Brooklyn  is  both  important  and  very  difficult  of  solution.  The 
Imhoff  tank  showed  a  very  gratifying  removal  both  of  bacteria 
and  suspensa  under  the  conditions,  but  the  size  of  tanks  required 
to  treat  the  storm  flow  would  make  the  tax  payers  sit  up  and 
take  notice.  A  plain  sedimentation  tank  would  probably  have 
done  the  same  work. 


Settling  Tank  3,  and  Separate  Digestion 

Settling  tank  3  (Dortmund  type)  was  8  ft.  x  8  ft.  in  plan, 
with  a  vertical  depth  of  4  ft.,  and  a  hopper  bottom,  having  its 
apex  line  3.6  ft.  making  the  greatest  total  depth  7.6  ft.  Its  water 
line  was  elevation  7.80. 

The  flow,  entering  thru  a  pipe,  was  carried  down  near  the 
center,  and  discharged  into  the  tank  5  ft.  below  the  water  line. 
The  effluent  was  taken  off  thru  V-shaped  notches  in  the  sides  of 
the  outfall  trough,  that  surrounds  the  top  on  all  four  sides'.  Eight 
notches  were  provided,  two  on  each   side.     A  6-in.  sludge-dis- 


Sewage  Treatment  Experiments 

i 


39 


E  - 


charge  pipe  was  placed  in  the  center,  ending  with  a  bell  at  the 
bottom,  and  having  a  clean-out  at  the  top  above  the  water  line. 
Sludge  was  discharged  from  this  pipe  thru  a  horizontal  branch 
placed  about  2  ft.  below  the  water  line  and,  passing  thru  the 
south  wall  of  the  tank,  discharged  thru  a  gate-valve  into  the 
flume  leading  to  the  sludge-drying  beds,  etc. 

Before  passing  thru  the  wall,  this  horizontal  portion  of  the 
sludge  pipe  gave  off  a  branch,  consisting  of  4-inch  pipe  controlled 
by  a  gate-valve,  which  passed  to  a  sludge-measuring  tank,  into 
which  the  daily  settlings  and  sludge  deposited  in  the  hopper  could 
be  discharged  and  measured,  and  from  which  they  could  be  fed 
slowly  by  gravity  into  the  sludge  digestion  tank.     (See  Fig.  13). 

The  "digestion  tank",  or  digesting  tank,  was  of  steel  plate, 
made  to  be  air  and  water-tight,  5  ft.  in  diameter  and  15  ft.  deep, 
with  a  conical  bottom.     It  was  set  vertically  in  the  ground,  the 


40 


Brooklyn,  N.  Y. 


top  of  the  tank  being  at  elevation  -f-  8.00.  The  lower  portion  was 
provided  with  an  outside  shell,  with  an  air  space  between  it  and 
the  tank. 

A  sludge-discharge  pipe,  and  an  overflow  pipe  for  water,  also 
were  provided ;  a  "clean  out"  for  the  sludge  pipe,  and  a  manhole 
in  the  top  that  could  be  sealed  airtight.  To  test  the  gases  pro- 
duced in  digestion,  a  small  valve  was  put  in  the  top. 

Operation  began  with  filling  the  tank  with  tap  water  to  eleva- 
tion -|-  6.50,  the  overflow  level.  Sludge  having  been  passed  from 
the  settling  tank  hopper  to  the  measuring  tank,  where  test  samples 
were  taken,  and  the  quantity  measured,  was  then  permitted  to 
enter  the  digestion  tank,  the  gate  valve  on  the  overflow  pipe 
being  opened,  allowing  the  water  displaced  by  the  entering  sludge 
to  escape. 

The  operation  of  this  tank  was  entirely  successful ;  never  at 
any  time  were  the  gases  given  off  offensive ;  the  surplus  water 
escaping  thru  the  overflow  when  the  tank  received  its  charge  of 
sludge  never  carried  offensive  odors.  The  gases  given  off  thru 
the  valve  at  the  top  were  odorless,  and  burned  with  a  colorless 
flame  when  ignited,  consisting  mostly  of  methane  gas. 


Fig.  14.     Sludge  Digestion  Tank. 

This  is  a  method  of  doing  the  work  of  the  Imhoff  Tank  in  two 
separate  tanks  and  is  remarkably  promising.  The  sewage  settles  in 
a  plain  sedimentation  tank  and  the  settlings  are  drawn  daily  under 
the  hydraulic  head  of  the  settling  tank  and  discharged  into  the  diges- 
tion tank ;  supernatant  water  being  let  out  of  the  overflow  to  provide 
room  and  draw  in  the  sludge  and  settlings.  The  digested  sludge  is 
dried  on  Imhoff  drying  beds,  and  is  of  the  same  quality  as  Imhoff 
Tank  sludge. 


Sewage  Treatment  Experiments  41 

Sludge  ripened  in  this  tank  perfectly,  and  could  not  have  been 
distinguished  from  Imhoff-tank  ripe  sludge.  It  was  full  of  gas 
bubbles  and  dried  readily  on  sludge  beds. 

The  operating  depth  of  the  tank  was  13.5  ft.  The  net  cubic 
capacity  for  digesting  sludge  was  265  cu.  ft. 

Settling  tank  3  had  a  net  capacity  285  cu.  ft.,  and  working  at 
a  theoretical  2-hours  retention  treated  25,584  gallons  of  sewage 
per  day,  equal  to  the  sewage  of  250  people,  the  digestion  tank 
thus  affording  only  a  space  of  1.06  cu.  ft.  per  capita,  which  with 
this  arrangement  of  tank  appeared  to  be  sufficient  for  producing 
a  good,  ripe  sludge.  This  is  much  less  space  than  proved  neces- 
sary in  the  Imhoff  tanks  themselves,  and  did  not  allow  much 
extra  room  for  sludge  storage.  As  soon  as  ripe  sludge  appeared 
in  the  hopper  it  was  discharged  and  dried.  It  is  very  doubtful 
if  a  large  tank  of  this  kind  would  be  successful  with  less  sludge 
capacity  per  capita  than  the  Imhoff  tanks  require.  In  operating 
a  large  tank,  sludge  drying  would  be  much  more  troublesome  on 
account  of  the  quantity  of  sludge.  A  small  amount  of  ripe  sludge 
takes  little  room  to  dry,  a  large  amount  takes  a  great  deal  of 
room,  and  drying  can  only  be  done  in  favorable  weather. 

It  is  possible  that  in  the  combination  of  a  settling  tank  with  a 
separate  digestion  tank,  less  space  is  required  for  digestion  than 
is  the  case  in  the  digesting  chamber  of  a  two  story  tank  because 
of  the  fact  that  raw  sludge  is  discharged  into  the  digestion  tank 
at  intervals  of  12  to  24  hours,  instead  of  continually ;  the  settling 
tank  in  this  case  doing  part  of  the  work  of  the  digesting  chamber-; 
this  at  least  seems  to  be  a  fair  inference  from  the  writer's  ex- 
perience. 

Ripening  of  sludge  in  the  separate  digestion  tank  was  much 
more  rapid  than  in  the  Imhoff  tanks.  On  being  started  the  tank 
was  seeded  with  a  few  gallons  of  ripe  Imhoff  sludge,  which  may 
have  helped.  Operation  began  late  in  March,  and  on  July  27, 
four  months  after  starting,  26  cu.  ft.  of  ripe  sludge  was  with- 
drawn. On  September  17  all  the  ripe  sludge  was  withdrawn,  and 
no  further  sludge  added  for  digesting,  for  several  months.  Dur- 
ing this  period  it  produced  in  ripe  sludge  up  to  the  time  of  the 
shut-off,  78  cu  ft,  and  later  the  unripe  sludge  remaining  after  the 
shut-down  digested  completely,  giving  20  cu.  ft.  more,  in  all  98 
cu.  ft.  The  ripe  sludge  contained  before  drying  94  per  cent 
moisture  and  dried  to  64  per  cent,  moisture ;  it  averaged  about  37 
per  cent,  volatile,  as  compared  with  the  Imhoff  sludge  which 
average  more  than  45  per  cent,  volatile. 

Effect  of  Depth  of  Tank  and  Capacity  of  Digestion  Chamber  on 
Quality  of  Imhoff  Sludge 

Accurate  information  on  the  best  depth  and  the  capacity  of 
digestion  chamber,  altho  of  extreme  importance  in  the  design  of 


42  Brooklyn,  N.  Y. 

a  permanent  sewage-treatment  plant,  is  unfortunately  very  diffi- 
cult to  obtain,  but  we  believe  that  the  results  secured  in  our  work 
cover  our  design  requirements. 

There  appeared  to  be  three  ways  of  securing  the  above  desired 
information.  First,  by  sounding  for  the  sludge  level ;  second,  by 
securing  samples  at  different  depths,  and  third,  by  observations  on 
the  behavior  of  the  tanks,  when  run  without  sludge  withdrawal, 
thruout  a  period  believed  to  represent  our  sludge-storage  require- 
ments, considered  in  connection  with  average  data  on  the  char- 
acter of  the  sludge  subsequently  withdrawn. 

Determination  of  the  sludge  level  by  means  of  soundings  was 
at  all  times  subject  to  uncertainty.  The  same  may  be  said  of 
the  method  of  obtaining  samples  at  different  depths.  Sounding 
and  sampling  could  only  be  performed  thru  the  gas  vents,  the 
shape  of  which,  together  with  that  of  the  tank  bottom,  interfered 
greatly  with  these  methods.  Even  if  the  design  had  not  inter- 
fered with  these  methods,  the  phenomena  observed  in  the  tanks 
rendered  the  observations  nearly  useless.  The  ebullition  of  gas 
was  so  active,  and  caused  so  much  circulation  in  the  contents, 
that  the  upper  portion  of  the  sludge  was  disturbed  and  kept  very 
fluid.  The  more  successful  the  tank,  the  more  difficult  was  the 
operation  of  finding  the  sludge  level.  Even  the  ripe  sludge 
ready  for  withdrawal  was  so  fluid  that  a  sounding  appliance 
readily  sank  thru  it.  The  most  that  can  be  said  from  these  ob- 
servations is  that  there  is  a  gradual  increase  in  the  density  of  the 
sludge  from  the  level  of  the  slots  to  the  bottom  of  the  tank.  This, 
at  least,  was  the  experience  at  Brooklyn. 

This  condition  has  been '  described  by  other  observers.  At 
Fitchburg,  it  was  found  that  the  best  way  to  ascertain  the  amount 
of  sludge  present  was  to  use  a  pump  with  a  hose  for  a  suction 
line,  which  was  lowered  slowly,  and  the  contents  of  the  tank,  thus 
removed  from  different  depths,  gave  the  information  sought.  It 
is  stated  in  the  report  describing  the  experiments  at  Philadelphia 
concerning  the  Imhoff  tank  that  "one  of  the  mechanical  difficul- 
ties was  the  inability  to  determine  at  what  level  the  sludge  stood 
in  the  digestion  chamber,  and  altho  sludge  was  withdrawn  in 
small  quantities  at  frequent  intervals,  it  is  now  believed  that  at 
times  it  was  allowed  to  reach  too  high  a  level,  so  that  the  ebulli- 
tion of  gas  forced  it  into  the  settling  portion  of  the  tank  to  the 
serious  detriment  of  the  effluent." 

The  third  method  gave  more  acceptable  data  than  either  of 
the  foregoing.  A  long  run  was  made  for  the  purpose  of  deter- 
mining the  effect  of  depth  of  tank  and  capacity  of  sludge  cham- 
ber on  quality  of  sludge.  Beginning  on  October  22,  1914,  the 
tanks  were  operated  at  2-hour  retention  without  withdrawing 
sludge  until  June  22,  1915,  when  sludge  had  reached  the  level  of 
the  slot  in  tank  2,  and  the  effluent  showed  marked  deterioration. 
The  tank  had  received  sewage  amounting  to  100,000  gallons  per 


Sewage  Treatment  Experiments  43 

day  thruout  the  period  of  observation.  The  inhabitants  contribut- 
ing sewage  were  estimated  at  1,000.  As  this  period  covered  about 
one  month  more  than  the  assumed  non-drying  period,  it  was  con- 
cluded that  the  storage  capacity  without  reaching  the  danger 
point,  would  have  been  about  correct  for  the  contributing  popu- 
lation. 

As  the  capacity  of  the  digestion  chamber  below  the  slots  is 
1,317  cubic  feet,  this  gives  1.317  cubic  feet  per  capita.  Tank  1 
received  145,000,  and  tank  3  received  50,000  gallons  of  sewage 
per  day  during  the  same  period.  At  the  time  that  the  sludge  in 
Tank  2  had  reached  the  danger  point,  that  in  Tank  1  was  ap- 
parently far  below  this  point,  thus  showing  the  sludge  storage 
capacity  of  Tank  1  was  greater  than  necessary.  Tank  3  had  not 
reached  the  danger  point  at  this  time,  but  this  tank  was  not  con- 
sidered as  reliable  as  Tank  2  for  the  determination  of  the  best 
capacity  of  digestion  chamber.  Tank  3  was  subject  to  the  diffi- 
culty that  unripe  sludge  was  at  times  discharge  by  "puncture." 
This  term  refers  to  the  following  phenomenon :  the  ripe  sludge 
occupied  so  small  a  depth  usually,  that  the  overlying  unripe  sludge 
would  at  times  be  drawn  into  the  sludge-pipe  together  with  the 
ripe.  While  the  proportions  of  the  settling  compartment  of  this 
tank  were  the  most  favorable  for  sedimentation  of  any  of  the 
tanks,  those  of  the  digestion  compartment  were  the  least  so,  and 
the  reason  appeared  to  be  because  the  cross-sectional  area  was  evi- 
dently too  great  for  the  depth.  This  rendered  the  observations 
for  sludge-chamber  capacity  of  less  value  than  those  on  Tanks 
1  and  2. 

It  will  be  seen  by  reference  to  Table  XXII,  that  the  sludge  in 
the  three  Imhoff  tanks  had,  on  the  average,  the  following  solid 
content:  Tank  1,  8.9  per  cent;  Tank  2,  8.9  per  cent;  Tank  3, 
6.7  per  cent.  As  related  to  the  sludge-storage  capacity,  it  will  be 
noted  that  the  volume  occupied  by  any  particular  weight  of  dry 
solids  in  sludge  is  inversely  in  proportion  to  the  percentage  of 
solids,  so  that  the  per  capita  yield  of  dry  solids  in  Tank  3  will 
occupy  a  volume,  8.9  divided  by  6,  equal  to  1.3  times  the  volume 
occupied  by  the  per  capita  yield  in  the  other  two  tanks.  Thus  if 
1.33  cu.  ft.  per  capita  be  the  allowance  for  a  tank  20  ft.  deep, 
1.73  would  have  to  be  allowed  for  one  14  ft.  deep,  owing  to  the 
greater  water  content  of  the  sludge. 

This  may  be  studied  in  a  slightly  different  manner.  If  the 
average  solid  content  be  obtained  as  the  result  of  combining  in- 
dividual figures  weighted  with  the  amount  of  sludge  at  the  par- 
ticular draft,  the  solid  contents  are  as  follows:  Tank  1,  8.8  per 
cent.;  Tank  2,  7.5  per  cent.;  Tank  3,  5.6  per  cent.  Calculating 
the  volumes  as  above,  a  per  capita  sludge-chamber  allowance  of 
1.33  cu.  ft.  in  a  20-ft.  tank  would  be  equivalent  to  1.12  cu.  ft.  in  a 
30-ft.  tank,  and  to  1.86  cu.  ft.  in  a  14-ft.  tank.  It  should  be  ob- 
served, however,  that  the  average  moisture  of  sludge  drawn  from 


H 


Brooklyn,  N.  Y. 


tanks  of  different  depths  may  be  to  a  considerable  degree  due 
to  the  by-passing  of  uncompacted  but  fairly  ripe  sludge  having  a 
larger  water  content  than  the  sludge  at  the  bottom. 

It  would  appear  that  the  water  content  of  ripe  sludge  is  largely 
affected  by  the  proportion  of  the  horizontal  cross  section  of  the 
tank  to  its  depth.  In  a  shallow  tank  with  a  wide  bottom,  the  layer 
of  ripe  sludge  is  comparatively  thinner  than  the  layer  of  similar 
sludge  in  a  deep  tank. 

The  required  drying  area  of  sludge  drying  beds  for  local  con- 
ditions was  found  to  be  0.38  square  feet  per  capita.  This  is 
subject  to  assumptions  as  follows:  That  all  drying  shall  be  done 
between  June  1  and  October  1,  and  that  time  is  required  for 
removal  of  dried  sludge  and  the  preparation  of  the  bed  for  its 
next  sludge  application ;  that  time  should  be  allowed  for  stormy 
periods  not  infrequently  met  during  the  summer.  The  depth  of 
the  wet  sludge  applied  at  a  time  is  taken  as  not  to  exceed  9  inches. 
Observations  lead  us  to  conclude  that  there  should  not  be  less 
than  ^2  sq.  ft.  per  capita.  The  best  medium  for  the  sludge  dry- 
ing beds  was  found  to  be  steam  ashes,  a  sand  covering  being  if 
not  undesirable,  at  least  unnecessary. 


Fig.   15.     Sludge   Drying   Beds,   from   top    of   Settling  Tanks. 


The  digestion,  storage,  and  drying  of  sludge  was  at  no  time 
accompanied  by  nuisance,  even  at  the  time  when  drying  was  de- 
layed by  storms.  The  dried  sludge  was  found  suitable  for  the 
filling-in  of  the  low-lying  meadow,  and  showed  no  tendency  to 
further  putrefaction.  The  shrinkage  in  volume  occurring  during 
the  first  day  of  drying  amounted  to  about  60  per  cent.  The 
period  of  drying  was  4  to  7  days  in  good  weather.     In  stormy 


Sewage  Treatment  Experiments 


45 


weather  it  amounted  to  from  10  to  14  days.  When  ready  for 
removal- from  the  bed,  the  dry  sludge  was  friable  and  porous,  and 
occupied  a  volume  not  exceeding  25  per  cent,  of  its  wet  volume. 


Fig.  16.     Removing  Dry  Imhoff  Sludge. 

The  separate  digestion  tank  proved  entirely  practicable ;  the 
ripe  sludge  secured  from  it  had  all  the  best  qualities  of  ripe 
Imhoff-tank  sludge.  It  had,  however,  one  disadvantage  over  the 
ordinary  Imhoff  tank,  namely :  the  necessity  of  providing  a  man 
to  discharge  the  sludge  from  the  settling  tank  into  the  digestion 
tank.  For  satisfactory  operation,  this  should  be  done  at  least 
twice  daily,  and  in  a  large  plant  would  add  considerably  to  the 
expense  of  operation.  As  this  is  not  required  in  the  ordinary 
Imhoff  tank,  maintenance  charges  are  more  in  favor  of  the  latter. 

In  studying  tank  performance  and  the  results  of  these  obser- 
vations on  the  required  capacity  of  digestion  chambers,  the  writer 
and  his  assistants  came  to  the  conclusion  that  the  space  per  cap- 
ita as  determined  would  be  too  small  for  general  use  in  the  de- 
sign of  tanks  for  disposal  plants  to  be  operated  in  the  ordinary 
manner,  and  that  it  should  be  made  larger  to  avoid  the  danger  of 
foaming.  The  larger  the  digestion  chamber,  the  less  danger  of 
this  appeared  to  exist  in  these  tanks.  Therefore,  as  a  factor  of 
safety  it  was  concluded  that  50  per  cent,  should  be  added  to  the 
figures  obtained,  when  used  for  purposes  of  design,  giving  2 
cu.  ft.  per  capita  for  a  tank  20  ft.  deep.  The  smaller  the  settling 
chamber  the  less  danger  there  is  of  foaming.  Shortness  of  reten- 
tion, and  high  rate  of  operation  are  more  important  than  high  per- 
centage of  removal.  The  following  tables  give  the  results  obtained 
in  very  short  retention  periods  : 


46  Brooklyn,  N.  Y. 

TABLE  XVI 
Sewage  Supplied  Experimental  Plant  1914-1915 

CRUDE  SEWAGE                      Oct.  Nov.    ~ "  Dec.  Jan.  Feb.         „  Mch, 

Temp.    degs.    C 19.4  15.9  12.0  Ff6  11.0  13.3 

Settled  in   cone  c.c 1.4  1.6  2.3  2.0  2.4  2.0 

solids:  p. p.m. 

Total    suspended    p.p.m 127  158  201  175  219  203 

Settling  in   2   hr.   p.p.m 78  97  86 

Colloidal     p.p.m 97  122  117 

Volatile    p.p.m 101  126  155  142  168  155 

Volatile  settling,  2   hr.  p.p.m 60  67  59 

Volatile    colloidal    p.p.m 82  101  96 

oxygen:  p.p.m. 

Demand     p.p.m 210  278  216  199  213  272 

Dissolved     p.p.m 2.2  1.9  2.0  2.8  2.5  1.4 

Saturation    % 24  19  18  26  25  13 

Nitrites    p.p.m 0.13  0.28  0.37  0.33  0.13  0.31 

Nitrates     p.p.m 0.9  0.14  0.13  0.13  0.13  0.16 

oxygen:   p.p.m. 


Consumed, 
Consumed, 

unfiltered    p.p.m. 
filtered    p.p.m 

.     51 
..     35 

61 
39 

71 
44 

74 
48 

82 
52 

75 
50 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Temp.    degs.    C 15.7  18.0  20.6  22.7  22.5  23.0 

Settled  in  cone  c.c 1.9  1.8  1.6  1.5  1.3  2.3 

solids:   p.p.m. 

Total    suspended    p.p.m :...   197  192  180  147  135  205 

Settling  in   2   hrs.   p.p.m. 74  69  50  '    45  43  79 

Colloidal     p.p.m 123  123  130  102  92  126 

Volatile    p.p.m 149  142  139  103  102  164 

Volatile  settling,  2  hrs.  p.p.m 48  45  36  26  27  55 

Volatile    colloidal    p.p.m 101  97  103  77  75  109 

oxygen:  p.p.m. 

Demand     p.p.m 270  211  276  173  216  305 

Dissolved     p.p.m 0.7  1.7  1.0  0.7  0.5  0.1 

Saturation    % 7  18  11  8  6  1 

TABLE  XVII 
Performance  of  Imhoff  Tanks,  1914-1915 

IMHOFF  EFFLUENT 

TANK  1 Oct.  Nov.  Dec.  Jan.  Feb.  Mch. 

Temp.    degs.    C 17.5  16.4  10.1  9.8  11.1  12.1 

Settled  in  cone  c.c 0.5  0.7*  0.9  0.8  0.6  0.6 

solids:  p.p.m. 

Total    suspended    p.p.m 94  118  146  127  142  144 

Settling  in   2   hr.  p.p.m 31  32  38 

Colloidal    p.p.m 96  110  106 

Volatile    p.p.m 76  95  117  107  102  110 

Volatile  settling,   2   hr.  p.p.m 24  16  26 

Volatile   colloidal   p.p.m 83  86  84 

oxygen:  p.p.m. 

Demand    p.p.m 113  130  101  165  187  214 

Dissolved     p.p.m 0.3  0  2.1  3.1  1.4  0.6 

Saturation     % 3  0  19  27  13  6 

Consumed:  p.p.m. 

Unfiltered    p.p.m 45  58  62  60  68  68 

Filtered    p.p.m 31  40  40  42  47  48 

Retention    hr 2  2  2  2  2  2 


Apr.  May  June  July  Aug.  Sept. 


Temp.    degs.    C 15.9  18.1  21.0  23.0  23.9  24.0 

Settled  in  cone  c.c 0.7  0.8  0.6  0.5  0.5  0.7 

solids:  p.p.m. 

Total    suspended    p.p.m 157  154  130  112  102  122 

Settling  in   2   hr.   p.p.m 40  40  26  26  28  31 

Colloidal    p.p.m 117  114  104  84  74  91 

Volatile    p.p.m 123  118  104  82  78  95 

Volatile  settling,   2   hr.   p.p.m 27  25  19  17  15  17 

Volatile  colloidal   p.p.m 96  93  85  65  63  78 

oxygen:  p.p.m. 

Demand     p.p.m 225  197  160  135  146  244 

Dissolved     p.p.m 0.3  0.4  0  0  0.1  0 

Saturation    % 3  4  0  0  10 

Retention    hr 2  2  2  2  2  2 


Sewage  Treatment  Experiments  47 

TABLE  XVIII 

Performance   of  Imhoff   Tanks,  1914-1915 


IMHOFF  EFFLUENT 

TANK  2  Oct.  Nov.  Dec.  Jan.  Feb.  Mch. 


Temp.    degs.    C 17.5              16.1                9.8                9.0  10.9  12.1 

Settled   in   cone  c.c 0.5               0.5                0.7                0.7                0.5  0.5 

solids:  p. p.m. 

Total    suspended    p.p.m 91               112               132  118  134  134 

Settling  in   2   hrs.   p.p.m.. 30  30  33 

Colloidal    p.p.m 88  104  ,     101 

Volatile    p.p.m 75                 95               110                 98  108  105 

Volatile  settling,  2  hrs.  p.p.m. 24  22      •  21 

Volatile  colloidal  p.p.m 74  86  84 

OXYGEN : 

Demand    p.p.m 66                90                80  136  156  207 

Dissolved    p.p.m 0.3               0                  1.5               2.7               1.0  0.3 

Saturation     % 3                  0                13                24  10  3 

Consumed:  p.p.m. 

Unfiltered    p.p.m 47                 57                 57                 58  63  64 

Filtered   o.p.m 33                 41                 38                 42  46  48 

Retention     hrs 2 2 2 2 2 2_ 

Apr.  May  June  July  Aug. Sept. 


Temp.    degs.    C 15.7  17.7  20.7  23.0  23.6  23.8 

Settled   in    cone   c.c 0.6  0.5  0.5  1.2  0.8  0.9 

solids:  p.p.m. 

Total    suspended    p.p.m ."..   134  125  153  179  113  123 

Settling  in  2   hrs.  p.p.m 30  24  28  54  35  34 

Colloidal    p.p.m 104  101  125  125  7i  89 

Volatile    p.p.m 106  98  123  100  106  82 

Volatile  settling,  2  hrs.  p.p.m 24  19  14  22  26  15 

Volatile  colloidal  p.p.m 87  84  105  78  80  67 

OXYGEN : 

Demand  p.p.m 211  200  223  199  129  222 

Dissolved    p.p.m 0.2  0.2  0  0  0.2  0 

Saturation    % 2  2  0  0  2  0 

Retention     hrs 2      2 2 2 2 2__ 

TABLE  XIX 

Performance   of  Imhoff   Tanks,  1914-1915 

IMHOFF  EFFLUENT 
TANK  3 Oct Nov.  Dec.  Jan.  Feb. __  Mch . 

Temp.    degs.    C 17.6  16.0  10.0  8.6  11.1  11.7 

Settled  in  cone  c.c 0.4  0.4  0.5  0.5  0.3  0.4 

solids:  p.p.m. 

Total    suspended    p.p.m 82  96  112  109  116  118 

Settling  in   2   hr.   p.p.m 26  23  20 

Colloidal   p.p.m 83  92  98 

Volatile    p.p.m 69  80  '    94  88  93  94 

Volatile  settling,  2  hr.  p.p.m 18  12  12 

Volatile    colloidal    p.p.m 70  81  82 

OXYGEN : 

Demand    p.p.m 106  149  109  135  159  171 

Dissolved    p.p.m 0  0  0.3  1.3  0  0 

Saturation    % 0  0  3  11  0  0 

Consumed: 

Unfiltered    p.p.m 45  54  51  50  56  60 

Filtered    p.p.m 35  39  34  37  41  44 

Retention    hr 2  2  2  2  2  2 


Apr.  May  June  July  Aug.  Sept. 


Temp.    degs.    C 15.7  18.8  20.7  22.6  23.6  23.2 

Settled  in  cone  c.c 1.4  1.1  0.9  0.6  0.7  0.4 

solids:  p.p.m. 

Total    suspended    p.p.m 142  149  139  115  106  96 

Settling  in   2   hr.   p.p.m 61  41  40  28  28  23 

Colloidal    p.p.m 121  108  99  87  78  73 

Volatile    p.p.m 139  112  111  82  82  75 

Volatile  settling,   2   hr.  p.p.m 40  25  28  15  18  12 

Volatile  colloidal  p.p.m 99  87  83            .    67  64  63. 

oxygen  : 

Demand    p.p.m 265  200  180  134  128  213 

Dissolved    p.p.m 0.6  0.4  0  0  0.3  0 

Saturation    % 6  4  0  0  3  0 

Retention     hr y2  .yA  1  1  3  3 


48 


Brooklyn,  N.  Y. 


TABLE  XX 

Performance  of  Settling   Tank   3,  1914-1915. 


Dec. 

EFFLUENT  FROM 

to 

SETTLING  TANK  3 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Apr. 

Closed 

Temp.  degs.  C. 

Settled  in  cone  c.c 0.7 

solids:  p.p.m. 

Total    suspended    p.p.m 103 

Settling  in  2    hr.  p.p.m 

Colloidal    p.p.m 

Volatile   p.p.m 81 

Volatile  settling  in   2  hr.  p.p.m 

Volatile  colloidal   p.p.m 

oxygen  consumed: 

Unfiltered    p.p.m SO 

Filtered    p.p.m 40 

Retention    hr 2 

Apr. 

Temp.    degs.    C 

Settled  in  cone  c.c 0.5 

solids:  p.p.m. 

Total    suspended    p.p.m 131 

Settling  in  2  hr.  p.p.m 29 

Colloidal    p.p.m 102 

Volatile   p.p.m 108 

Volatile  settling,   2   hr.   p.p.m 23 

Volatile  colloidal   p.p.m 85 

Oxygen  demand   p.p.m 

Retention    hr 2 


69 


51 

41 
2 


0.3 
93 

"76" 


0.4 
92 


May 


June 


July 


0.5 
118 
.„„... 


48  50  64 

35  35  43 

2  2  2 


Aug. 


Sept. 


0.4 

0.2 

0.2 

0.2 

0.2 

113 

108 

100 

96 

103 

22 

14 

18 

13 

12 

91 

94 

82 

83 

91 

93 

88 

77 

78 

84 

16 

11 

12 

8 

6 

77 

77 
200 

65 

70 

78 
180 

2 

2 

2 

2 

2 

TABLE  XXI 

Removals  Effected  by  Tanks,  1914-1915. 

Per  Cent,  of  Reduction  Effected 


Imhoff  Tanks 
1  2 


Settling 
Tank  3 


volumetric: 

Solids  settleable   in  Imhoff   cone 62 

GRAVIMETRIC : 

Total   suspended   solids 27 

Total    settleable   solids 53 

Total   colloidal    solids 26 

Total   volatile  solids 31 

Oxygen    demand 29 

Retention  in  tanks,  hr 2 


64 


78 


80 


27 

40 

40 

62 

66 

61 

12 

21 

21 

31 

46 

40 

34 

41 

20 

2 

2 

2 

TABLE  XXII 
Summary  of  Sludge  Drying  Data 


l 


% 

Moisture  before  drying 91.1 

Moisture   after  drying 67.8 

Volatile    solids 45. S 

Average  depth  on  beds,  inches 9. 

Average  number  of  days  drying 7.1 


Imhoff  Tanks 

Separate 

2 

3 

Dig.  Tank 

% 

% 

% 

91.1 

93.3 

94.6 

66.2 

69.7  . 

64.1 

46.5 

46.7 

36.6 

9. 

9 

10 

7.3 

7.1 

7.3 

Sewage  Treatment  Experiments  49 

The  Riensch-Wurl  Screens. 

The  fine  screens  from  which  the  data  following  were  obtained 
are  two  in  number,  of  the  Riensch-Wurl  design,  installed  at  the 
26th  ward  treatment  plant  as  a  permanent  part  of  that  plant. 
They  were  completed  and  commenced  operation  early  in  1916 
and  since  then  havQ  been  in  service. 

The  Riensch-Wurl  screen  consists  principally  of  a  disc  of 
perforated  metal  plate,  built  in  sections  on  a  steel  frame,  with 
the  frustum  of  a  cone  built  up  from  its  center,  the  whole  being 
mounted  on  a  shaft  whose  inclination  from  the  vertical  causes  the 
disc  to  tilt  at  an  angle  with  the  horizon.  A  close  seal  is  provided 
between  the  outer  edge  of  the  rotating  disc  and  the  stationary 
setting  in  which  it  is  placed.  To  secure  this  a  Z-bar  is  embedded 
in  the  masonry  with  one  of  its  flanges  exactly  in  the  plane  of  the 
top  of  the  rim  of  the  frame  or  setting.  The  Z-bar  carries  on  its 
top  a  large  number  of  small  segments,  which  can  be  moved  in 
and  out,  so  that  the  distance  between  their  edges  and  that  of  the 
disc  is  not  greater  than  the  width  of  the  slits  or  apertures  in  the 
disc.  When  these  segments  are  adjusted,  they  are  held  in  posi- 
tion by  bolts. 

The  plane  of  the  disc  as  designed  in  the  Brooklyn  screens  is 
inclined  25  degrees  with  the  horizon,  and  about  2/3  of  its  surface 
is  submerged  in  the  sewage,  while  the  other  1/3  is  above  the 
sewage  surface.  As  the  disc  revolves,  the  screenings  are  brought 
above  the  sewage  and  remove  by  means  of  revolving  brushes 
carried  at  the  ends  of  the  arms  of  a  large  spider.  The  brushes 
revolve,  as  well  as  the  arms  and  the  disc,  and  the  combined  result 
is  that  every  portion  of  the  surface  of  the  screen  is  brushed  over 
at  least  twice  during  each  revolution  of  the  disc ;  except  that  the 
conical  portion,  which. is  cleaned  by  means  of  a  vertical  brush, 
gets  but  one  brushing. 

The  design  of  the  plant  provides  for  a  by-pass  between  the 
screens  so  that  they  may  be  operated  individually  or  in  series,  and 
double  screening  tried  out.  Double  screening  did  not  prove  to  be 
advantageous.  The  screens  are  fourteen  feet  in  diameter  and 
operated  by  steam-engine  drive. 

The  contract  provided  that  the  apertures  or  slits  thru  which 
the  screening  is  effected,  should  be  made  of  such  size  as  might 
be  found  on  trial  to  give  the  best  results  with  the  average  flow  of 
sewage  at  this  location.  Screening  practice  and  experience  else- 
where indicated  that  this  should  be  determined  experimentally, 
and  with  this  end  in  view  the  specifications  provided  that  four 
complete  sets  of  screen-surface  plates  be'  supplied,  as  follows : 
The  first  cut  with  apertures  5/64-inch  wide,  the  second  with 
apertures  1/16-inch  wide,  the  third  with  apertures  3/64-inch 
wide,  and  the  fourth  with  apertures  1/64-inch  wide.  In  each  set 
the  apertures  were  two  inches  long  and  staggered  in  the  bronze 


50  Brooklyn,  N.  Y. 

plate,  which  is  3^-inch  thick,  each  aperture  having  a  counter- 
sunk cross-section,  with  the  narrow  part  of  the  opening  on  the 
face  of  the  screen.  The  1/64-inch  proved  too  fine  and  was  soon 
recut  to  1/32  inch. 

These  sets  of  screen  plates  were  mounted  on  the  screen  frames 
successively,  and  tried  out,  and  the  aperture  dimension  1/16-inch, 
which  was  decided  to  be  the  most  suitable  for  screening  the  sew- 
age at  this  plant,  was  selected  for  permanent  installation.  The 
plates  not  selected  remained  the  property  of  the  contractor. 

It  was  provided  that  the  size  and  number  of  apertures  must 
be  such  that  6,000,000  gallons  of  sewage  would  pass  thru  the 
screen  in  24  hours,  the  difference  of  head  of  sewage  entering  and 
leaving  being  not  more  than  12  inches,  and  that  the  screen  must 
remove  from  the  sewage  practically  all  particles  of  suspended 
matter  with  a  diameter  50  per  cent,  greater  than  the  cross-section 
of  the  aperture. 

The  screens  were  supplied  by  the  Sanitation  Corporation  of 
New  York,  and  the  design  for  placing  them  was  made  by  the 
engineers  of  that  corporation.  In  designing  the  plant,  the  Bureau 
of  Sewers  made  provision  for  the  installation  of  the  screens  in 
accordance  with  the  design  supplied.  The  screens  were  guaran- 
teed for  one  year  of  service,  at  the  end  of  which  they  should 
be  in  perfect  repair,  when  taken  over  by  the  city.  No  require- 
ment was  made  as  to  the  amount,  or  per  cent.,  of  removal  which 
should  be  made  from  the  sewage,  as  it  was  believed  that  how- 
ever little  or  much  this  might  prove  to  be,  the  screens  were  neces- 
sary to  help  out  the  existing  sewage  treatment  plant,  then  at- 
tempting to  treat  four  times  more  sewage  than  it  could  fairly 
take  care  of.  As  it  was  not  known  what  fineness  of  screens  was 
best  for  local  conditions,  this  was  made  the  subject  of  experiment 
as  already  mentioned.  The  experimental  work  was  placed  in 
charge  of  the  experiment  station. 

In  designing  sewage-screening  plants,  one  has  but  little  in- 
formation to  go  on.  Data  are  as  yet  very  meager.  Sewages 
differ  very  widely  in  the  character  of  suspended  solids,  and 
naturally  the  kind  of  screens  which  will  serve  best  with  a  given 
sewage,  and  the  required  fineness  of  the  screening  surface,  vary 
widely.  In  Dresden,  Germany,  where  there  are  four  large  screens 
of  the  Riensch-Wurl  design,  the  breadth  of  the  apertures  is  2 
m.m.,  which  is  found  at  that  place  to  give  a  sufficient  removal  of 
suspended  matter,  and  this  is  the  only  treatment  found  necessary 
for  the  sewage  of  that  city,  which,  after  screening,  is  discharged 
thru  multiple  outlets  directly  into  the  Elbe  River,  apparently 
affording  a  satisfactory  disposal  of  sewage.  Dresden  has  a  pop- 
ulation of  between  400,000  and  500,000,  and  at  its  lowest  stages 
the  river  is  comparatively  a  small  stream.  The  writer  was  told 
in  Dresden  that  slits  or  apertures  of  less  width  than  2  m.m. 
reduce  the  capacity  of  the  screens  without  much  added  advantage. 


Sewage  Treatment  Experiments  •  51 

This  is  especially  the  case  where,  from  the  nature  of  the  sus- 
pended matter,  a  mat  tends  to  form  over  the  screen  surface  and 
produce  a  straining  effect.  In  Dresden  the  difference  of  head 
thru  the  screens  varies  from  2  to  6  c.  m.,  under  ordinary  condi- 
tions, but  at  times  is  much  greater  than  the  larger  figure — possi- 
bly four  times  as  much. 

The  two  Brooklyn  screens  were  installed  early  in  April,  1916. 
The  north  screen  was  equipped  at  this  time  with  screen  plates 
perforated  with  slits  2  inches  long  and  5/64-inch  wide.  The 
south  screen  was  equipped  with  plates  with  slits  2  inches  long 
and  1/16-inch  wide. 

The  following  tables  give  the  results  obtained  at  this  time  on 
dry  weather  flow  sewage  on  the  official  test : 

TABLE  XXIII 
Operation  of  Riensch-Wurl  Screens 

April-May,    1916 
Capacity  and  Head  Developed 


Screen 

Slit 
inches 

Sewage  Screened       Average  Loss  of  Head 
in  24  hrs. — gals.          at  Screen — inches 

North    

South     

5/64 
1/16 

8,077,851                          0.118 
8,204,113                          0.121 

Removal  Effected 

as  Shown 

by 

Crude  Sewage  and  Effluent 

Crude  Sewage  Screen  Effluent  Removal 

North  Screen —                                    p. p.m.  p.p.m.  p.p.m.  % 

Suspended   solids 159  137  22  14.0 

Settling    solids 75  60  15  20. 

Non-settling    84  77  7                            8.7 

South  Screen — 

Suspended    solids 193  158  35  18.0 

Settling    solids 109.5  79.5  30  27.4 

Non-settling    83.5  78.5  5                           6.0 

The  screens  continued  in  operation  with  5/64-inch  and  4/64- 
inch  slits  until  the  spring  of  1917. 

The  following  table  gives  a  fair  picture  of  the  operation  of 
the  screens  in  comparison  with  the  Imhoff  tanks.  The  data  are 
averages  from  all  of  the  tanks  taken  together,  and  of  both 
screens  averaged.  The  crude-sewage  influents  are  also  averaged. 
The  table  includes  dry  weather  flow  only,  storm  water  data  as 
far  as  possible  are  excluded,  and  also  all  abnormal  data.  Screen 
removal  was  at  times  subject  to  wide  variations  and  to  extremes, 
these  are  excluded  as  far  as  possible. 


52         •  Brooklyn,  N.  Y. 

TABLE  XXIV 


Sewage  Imhoff  Tanks  R.  W.  Screens 

Inf't.  Eff't.  %  Red.  Eff't.  %  Red. 


Solids,  by  cone,  c.c.l. l.S 

Solids,    susp.    tot.    p.p.m 171. 

Solids,    settlings    p.p.m 61 

Solids,    colloid    p.p.m 110 

Solids,    volatile   p.p.m 133 

Solids,   vol.    settling   p.p.m 41 

Solids,   vol.   colloid   p.p.m 92 

Oxygen    demand    p.p.m 259 


0.6 

66 

0.9 

50 

103. 

40 

137 

20 

29 

52 

45 

26 

81 

26 

98 

11 

82 

39 

107 

19 

17 

59 

73 

21 

152 

41 

182 

30 

Theoretic  retention,  all  tanks,  2  hours. 

Sewage  passing   tanks  per  day: 

Tank     1 145,000  gallons 

Tank    2 100,000 

Tank     3 50,000 


295,000   gallons 

Sewage    screened    (both    screens) 12   Mgd. 

North  screen 5/64  in.  aperture 

South   screen 4/64  in.  aperture 

Screenings    removed    (both    screens) 14.75  cu.  ft.  per  Mg. 

Weight  per  cu.  ft 72  lbs. 

Moisture  81   % 

In  preparing  the  above  table  much  discrimination  has  been 
called  for,  that  the  presentation  might  be  fair.  Storm  removals 
often  go  above  50  per  cent  of  the  suspensa.  Dry  weather  removal 
sometimes  falls  nearly  to  0.  These  extremes  mislead.  Average 
performance  has  been  sought  for.  To  obtain,  as  well  as  select 
these  data  has  been  very  difficult. 

To  include  the  abnormal  is  to  render  all  conclusions  useless. 
It  should  be  remembered  that  screens  always  remove  the  screena- 
ble  solids, — if  these  are  absent  they  remove  100  per  cent,  of 
nothing. 

In  1917  the  following  changes  were  made  in  the  screens:  The 
north  screen  was  changed  to  a  screening  surface  with  apertures 
3/64-inch  wide,  and  the  south  screen  to  a  surface  with  apertures 
1/32-inch  wide.     In  both  cases  the  apertures  were  2  inches  long. 

The  weather  was  very  unfavorable  for  making  observations 
on  screening  dry  weather  flow,  on  account  of  the  prevalence  of 
frequent  rainy  days  and  showers.  The  screens  were  designed  for 
receiving  both  dry-weather  flow  and  storm  water.  It  was  difficult 
to  obtain  satisfactory  data  on  dry-weather  flow,  and  many  ob- 
tained were  useless  on  account  of  storm  interference. 

A  remarkable  difference  was  observed  in  the  dry-weather 
sewage  discharged  into  the  quieting  tank,  from  which  the  Imhoff 
tanks  were  supplied,  and  that  which  entered  the  screens.  To 
study  this  phenomenon  occupied  several  weeks  of  valuable  time. 
Meanwhile  the  screens  operated  on  a  very  weak  sewage.  Un- 
fortunately the  sewer  and  grit  chamber  could  not  be  properly 
examined  for  obstructions  without  cutting  off  the  sewage  supply 
to  the  experimental  plant,  and  as  the  flow  to  the  Imhoff  tanks 
was  normal  in  quality  it  was  decided  not  to  do  this  until  the 
July-August-September   series  of   tests   were  compiled.      In  the 


Sewage  Treatment  Experiments 


53 


latter  part  of  September  the  pumps  were  stopped  and  the  sewer 
and  grit  chamber  cleaned ;  an  obstruction  was  then  found,  made 
of  tin  cans,  rags,  sticks,  stones,  silt,  etc.,  formed  diagonally  across 
the  chamber  above  the  entrances  to  the  screens,  where  it  had 
acted  as  a  submerged  weir,  interfering  greatly  with  the  dry- 
weather  flow  to  the  screens,  but  not  at  all  with  the  flow  to  the 
by-pass  from  which  sewage  was  obtained  for  the  tanks.  This 
did  not  much  interfere  with  the  storm  water  flow,  which  readily 
passed  over  it. 

The  screen  data  for  this  period  are  of  value  only  as  showing 
what  removal  might  be  expected  from  a  very  weak  sewage, — 
from  which  much  of  the  suspensa  had  settled.  Under  the  condi- 
tions much  more  effect  was  produced  on  the  sewage  than  would 
have  been  anticipated  in  advance. 

TABLE  XXV 

Operation  of  Imhoff  Tank  2,  on  Fairly  Strong  Sewage,  and  R. 

W .  Screens  on  Abnormally   Weak  Sezuage,  Compared 

Influent  to  Influents  to 

Crude  Sewage  _    Imhoff  Tank  2  South  Screen  North  Screen 

Temp.  deg.   C 26.  26\  26\ 

Solids  by  cone  c.c.l 1.9  1.4                             0.64 

Solids  susp.   total   p.p.m 183  136  119 

Solids  volatile   p.p.m 143  106  92 

Solids  mineral   p.p.m 40  30  27 

Dissolved    oxygen    p.p.m 0.5  1.0                               1.0 

Oxygen    demand    p.p.m 245  137  110 

Filtered   p.p.m. 144.  112  88 

Effluent  Effluent  R.  W.  Screens 

Effluents Imhoff  Tank  2 South  1/32"  North  3/64" 

Tank  retention,  2  hr.  %  Red  %  Red  %  Red 

Solids  by   cone  c.c.l 1.1  44  1.2  14  0.5  22 

Solids  susp.   total  p.p.m 138.  25  133  2  117  2 

Solids  volatile  p.p.m 109  24  108  ....  92  0 

Solids    mineral    p.p.m 30  25  25  18  25  4 

Dissolved   oxygen    p.p.m 0  ....  10  10 

Oxygen   demand   p.p.m 198  15  126  8  90  18 

Filtered    p.p.m 119 17 77  31  65  26 

South  Screen  North  Screen 

Sewage   screened   per   diem 6.4  Mgd.  8.4  Mgd. 

Average  amount  of  screenings  per  diem 13.2  cu.  ft.  13.9  cu.  ft. 

Average  amount  of  screenings  per  mg 2.06  cu.  ft.  1.65  cu.  ft. 

Average  weight  of  screenings  per  cu.  ft 72  lbs.  72  lbs. 

Computed  from  averages 

It  will  be  noted  that  the  sewage  entering  the  screens  was  in 
nearly  every  item  of  its  constituents  weaker  than  the  effluent 
leaving  the  Imhoff  tank.  It  may  be  presumed,  perhaps,  that  had 
the  effluent  from  the  tank  passed  thru  screens,  it  would  have  been 
improved.  The  volume  of  screenings  removed  from  such  a  weak 
sewage  is  worthy  of  remark. 

Unfortunately  the  screenings  produced  were  not  weighed.  The 
weights  given  in  the  table  are  an  average  weight  for  similar 
screenings  determined  by  a  number  of  experiments. 

After  the  sewers  were  cleaned  out,  the  following  data  were 
obtained,  the  screens  operating  as  above  on  dry  weather  sewage. 


54 


Brooklyn,  N.  Y. 


Imhoff  tank  2  being  also  supplied  with  the  same  sewage,  which 
was  not  as  strong  as  the  yearly  average  by  about  10  per  cent., 
but  was  not  to  be  considered  abnormal.  It  will  be  observed  that 
while  the  screen  improved  its  removal,  the  tank  showed  the 
effect  of  a  weaker  sewage  : 

TABLE  XXVI 

Oct.-Nov.-Dec,    1917 

Imhoff  Tank  2 
Sewage  2  Hrs.  Retention 

Inf't.  Eff't.  Rem'l.  %  Red 

Solids  by   cone  c.c.1 1.4                      0.8                    0.6  44 

Solids  susp.   total   p.p.m 141  116  25  18 

Solids  volatile  p.p.m 118  98  20  17 

Solids    mineral    p.p.m 23  18                       5  22 

R.  W.  Screens 

Sewage  given  above  South  Screen  1/32"  North  Screen  3/64" 

Efft.  Rem'l.       %  Red  Efft.  Rem'l.       %  Red 

Solids  by  cone  c.c.l 0.5  0.9  64  0.6  0.8  54 

Solids  susp.  total   p.p.m 116  25  18  117  24  17 

Solids   volatile   p.p.m 93  25  21  94  24  20 

Solids  mineral  p.p.m 21  2  9  22  1  4 

Sewage  screened   per  diem 5   Mgd.  5   Mgd. 

Screenings    per    diem 54  cu.   ft.  52   cu.   ft. 

Weight   per   cu.    ft 73.7  lbs. 74.2  lbs. 

Screened  sewage  was  applied  to  the  trickling-filter  beds  during 
the  period  beginning  July  1  and  ending  December  31,  1916,  with 
tl\e  object  of  comparing  the  results  obtained  with  those  from  Im- 
hoff-tank  effluent.  The  filter  beds  had  been  operated  with  Imhoff- 
tank  effluent  previously  to  receiving  the  screened  sewage.  They 
had  been  out  of  service  during  February  and  March,  but  at  the 
time  were  in  good  condition  tho  they  had  not  fully  regained  their 
purifying  power.  For  this  reason  and  in  order  that  the  effect  of 
the  Imhoff  effluent  might  have  time  to  pass  off,  the  results  of  the 
first  month  of  operation  are  not  included  in  the  table  which  fol- 
lows : 

TABLE  XXVII 

Trickling  Filter  No.  3,  Operated  on  Riensch-Warl  Screen  Effluent 

Aug. -Sept. -Oct.,    1916  Apertures   1/16   and   5/64-inch 


Sewage  Effluent  from  Filter  No.  3* 
Screened  Depth  of  Stone  Medium 
Average  4  ft.  6  ft.  7'3"  8'6" 

Temp.    degs.    C 19.4  18.7  18.5  17.7 

Suspended  solids, 

Total    p.p.m 153  92  86  84  114 

Volatile    p.p.m 110  85  61  61  79 

Diss,   oxygen,   p.p.m 3.1  3.7  3.6  4.1 

Per  cent,   of   sat 33  39  38  43 

Relative  stability, 

Unfiltered    per    cent 37  95  93  97 

Filtered    per    cent 70  100  100  100 

Oxygen  dem.  bioch. 

Unfiltered    p.p.m 129  72  40  44  40 

Filtered    70  30  19  18  16 

Nitrite  nitrogen  p.p.m 1.6  2.6  3.0  3.4 

Nitrate  nitrogen  p.p.m 1.1  4.1  3.8  4.6 

Oxidized  nitrogen  p.p.m 2.7  6.7  6.8  8.0 

*  Rate  of  application  4  Mgd.  per  acre. 
Size  of  filter  medium  1)4  to  2]/2  inches. 


10' 


From 

Humus 

Tank 


18.0 


114 

26 

79 

22 

4.3 

2.8 

45 

30 

99 

100 

100 

43 

23 

18 

00 

3.0 

3.2 

5.4 

5.7 

8.4 

8.9 

Sewage  Treatment  Experiments  55 

The  settled  effluent  from  the  10-foot  depth  was  uniformly 
stable,  and  the  results  on  filtered  samples  from  all  other  depths 
except  the  4-foot  indicated  that  after  settling  out  the  humus  the 
effluent  would  be  satisfactory. 

The  rates  of  operation  on  filters  Nos.  1,  2,  3  were  set  at  4 
mgd.  per  acre,  filter  No.  4  being  kept  at  the  rate  of  2  mgd.  These 
were  the  same  rates  as  had  been  employed  in  1915,  and  previously 
to  applying  the  screened  sewage,  except  that  filter  bed  No.  1  was 
increased  in  an  effort  to  test  it  to  destruction  as  regards  clogging. 

There  seemed  no  especial  tendency  to  clog  the  beds.  It  was 
threatened  on  bed  No.  1,  but  did  not  develop.  The  sewage  ap- 
plied to  the  filter  beds  was  at  all  times  fresher  than  Imhoff-tank 
effluent  would  have  been.  No  unpleasant  smells  were  at  any 
time  in  evidence. 

In  the  results  obtained  from  the  operation  of  the  Riensch- 
Wurl  screens  at  this  station,  it  is  to  be  observed  that  the  average 
percentage  removal  of  settling  matter  is  low.  When  the  sewage 
enters  the  screen  chamber  it  comes  in  with  a  rush  strong  enough 
to  force  many  of  the  finer  particles  thru  the  screen.  These  par- 
ticles tend  to  settle  behind  the  screen,  but  do  not  accumulate 
there,  as  they  are  carried  away  by  the  effluent.  The  samples  for 
analysis  taken  from  the  effluent  were  obtained  as  it  flows  out  into 
the  channel  leading  to  the  measuring  weir,  and  may  have  con- 
tained an  undue  proportion  of  the  particles  above  mentioned. 
It  is  difficult  to  obtain  a  sample  from  the  effluent  which  shall 
be  as  representative  of  the  work  done  as  a  sample  taken  from 
the  Imhoff-tank  effluent  would  be.  A  good  deal  of  solid  matter 
which  really  is  screenable  as  well  as  settleable  and  would  be  re- 
moved under  proper  conditions,  is  forced  thru  the  screens  by  the 
high  rate  of  approach. 

A  serious  difficulty  was  experienced  in  obtaining  reliable  re- 
sults on  the  work  of  the  fine  screens,  due  to  the  fineness  of  the 
analytical  methods  employed.  It  is  obvious  that  a  test  for  sus- 
pended solids  by  the  Gooch-crucible  method,  or  for  settling  mat- 
ter by  the  Imhoff  cone,  cannot  take  into  account  the  grosser  par- 
ticles of  matter  that  may  be  present  in  the  sewage,  lest  the  results 
be  totally  incongruous.  This  is  partly  remedied,  of  course,  by 
noting  the  time  required  to  fill  a  bucket  with  screenings,  and  the 
volume  of  sewage  observed.  The  bucket  takes  into  account  both 
the  fine  material  and  the  coarser  matter,  but  unfortunately  there 
seems  to  be  no  rational  way  of  crediting  the  screenings  removed 
to  the  removal  of  total  suspended  solids.  It  sometimes  happened 
that  by  the  determinations  according  to  Standard  Methods,  A.  P. 
H.  A.,  the  effluent  contained  more  suspended  matter  than  the 
influent,  notwithstanding  the  fact  that  the  screen  was  removing 
350  to  500  lbs.  of  screenings  per  million  gallons.  One  is  tempted 
to  ask  the  question,  after  this  experience,  are  our  methods  of 
determining  suspended   solids  above  improvement  or  criticism? 


56  Brooklyn,  N.  Y. 

If  we  do  not  obtain  correct  data  on  the  influent  to  the  screens, 
obviously  we  do  not  for  the  tanks  either.  Some  discussion  on  this 
point  seems  desirable. 

Cleaning  of  the  Screens 

Twice  a  day,  a  jet  of  water  from  a  hose  was  played  on  the 
revolving  disc.  The  water  struck  the  screen  with  considerable 
force,  and  the  slits  were  thoroly  cleaned  thereby.  This  treat- 
ment was  followed  by  pouring  about  four  ounces  of  kerosene 
on  each  of  the  four  revolving  brushes.  This  application  not  only 
removed  the  grease  from  the  brushes,  but  also  from  the  screens. 
In  this  way  it  was  possible  to  keep  the  screen  clean  and  bright  at 
all  times. 

Removal  of  Screenings 

When  a  bucket  was  filled  with  screenings,  it  was  removed  and 
an  empty  bucket  replaced.  The  time  was  recorded.  In  this  way  the 
time  required  for  filling  each  bucket  was  obtained.  The  full 
bucket  was  hoisted  by  a  pulley  and  emptied  into  a  special  car. 
The  car  was  rolled  to  the  dump  about  50  yards  away  and  emptied 
on  steam  ashes.  During  the  summer  this  dump  was  sometimes 
offensive,  and  became  infested  with  flies,  but  this  could  have  been 
prevented  at  all  times  by  applying  a  slight  amount  of  lime  and 
covering  with  a  thin  cover  of  soil  which  was  purposely  omitted 
at  this  time  in  order  that  the  tendency  to  cause  nuisance  might 
be  observed.  With  the  coming  of  cooler  weather,  tendency  to 
cause  nuisance  ended.  When  the  screenings  are  dry,  nuisance  is 
practically  absent.  It  is  desirable  to  treat  the  screenings  shortly 
after  their  removal,  so  that  all  nuisances  will  be  avoided.  Com- 
posting with  ordinary  earth  has  proved  sufficient  to  prevent  all 
nuisance.  Artificial  drying  and  incineration  are  recommended  as 
a  method  of  disposal. 

Samples  of  the  screenings  were  examined  for  moisture  con- 
tent, volatile  matter  and  ash.  The  volatile  matter  and  ash  were 
determined  on  the  dry  specimen.  Samples  for  analysis  were 
collected  in  an  8j^-inch  porcelain  dish,  and  the  test  for  moisture 
content  was  made  on  a  portion  of  this  sample  in  a  4-inch  evap- 
orating dish.  Care  was  taken  to  obtain  as  representative  a  sam- 
ple as  possible.  The  moisture  determination  was  made  by  evap- 
orating the  water  in  a  weighed  sample  on  a  water  bath,  for 
twenty  hours.  The  sample  was  then  further  dried  in  a  hot-air 
oven  at  100  deg.  C,  for  four  hours  more.  The  loss  in  weight 
was  determined  and  the  percentage  of  water  calculated.  The 
volatile  matter  was  determined  gravimetrically  by  igniting  a  care- 
fully weighed  portion,  in  a  Wiesnegg  muffle  furnace,  until  the  con- 
tents were  at  a  red  heat.  The  loss  in  weight  gave  the  volatile 
matter,  and  the  difference  from  the  original  weight  gave  the  ash. 


Sewage  Treatment  Experiments 


57 


The   following  table  gives   the   results   of    screenings    from   the 
south  screen.    Apertures  1/32-inch. 

Analyses  of  Screenings 

Moisture                Volatile  Matter  Ash 

Date                                       Per  Cent.                    Per  Cent.  Per  Cent. 

July    27,    1917 88.7                              87.8  12.2 

July   30,    1917 86.0                              81.6  18.4 

Aug.      1,    1917    (a) S9.0                              67.0  33.0 

Aug.      1,    1917    (b) 76.8                              68.3  31.7 

Aug.      7,    1917    (c) 66.4                              82.0  18.0 

Aug.    10,    1917 71.6                              82.0  18.0 

Aug.    IS,    1917 87.3                               74.8  25.2 

Averages    83.4                             81.6  18.4 

: f 

The  averages  exclude  the  three  special  samples. 

(a)  Sample  of  storm  screenings. 

(b)  Sample  of  screenings  taken  near  the  end  of  the  storm  of  August  1. 

(c)  Sample  taken  after  water  in  screenings  had  been  permitted  to  drain  off. 

Time  required  for  one  complete  revolution  of  screen   2.4   minutes.     Time  required   for  one 
complete  revolution  of  brushes  7.5  seconds. 

Results  from  the  north  screen  follow.    Apertures  3/64-inch. 


Moisture  Volatile  Matter  Ash 

Date  Per  Cent.  Per  Cent.  Per  Cent. 

Aug.    21,   1917 82.0  80.9  19.1 

Aug.    23,   1917 80.0  77.4  22.6 

Aug.    24,   1917 86.8  93.5  6.5 

Aug.    27,   1917. 89.5  95.6  4.4 

Aug.    31,1917 85.9  78.8  21.2 

Sept.     5,    1917 89.0  90.5  9.5 

Sept.    12,   1917 90.4  94.0  6.0 

Averages    86.3  87.2  12.8 

Time  required  for  one  complete  revolution  of  screen  2.3  minutes. 
Time  required  for  one  complete  revolution  of  brushes  7.8  seconds. 

It  will  be  seen  from  this  table  that  the  water  content  of  screen- 
ings is  generally  high.  Another  important  fact  is  the  relatively 
large  amount  of  organic  matter.  The  weight  of  the  screenings 
varied  from  64  to  80  lbs.  per  cu.  ft.  the  average  being  about  72 
lbs.,  which  is  rather  high ;  this  is  probably  due  to  the  age  and 
condition  of  the  sewage,  and  considerable  grit. 


Storm  Water  Operation  of  R.  W .  Screens 

It  was  evident  on  studying  the  data  obtained  that  storms  pre- 
sent such  varied  phenomena  that  averages  were  highly  mislead- 
ing. It  was  also  obvious  to  the  observers  that  the  ordinary  meth- 
ods of  sampling  and  examination  by  Gooch  crucible,  and  even 
by  the  Imhoff  cone,  were  of  little  or  no  value,  as  these  could  not 
possibly  give  any  correct  result  when  used  on  matters  of  such 
large  size  as  the  storm  water  brought  down — such  as  tin  cans, 
old  shoes,  rags  in  quantity,  as  well  as  all  grades  and  sizes  of 
suspended  solids.  It  was  concluded,  and  we  believe  correctly, 
that  the  best  means  of  exhibiting  storm-screen  performance  was 
to  give  the  amount  of  screenings  removed  by  the  screen,  and  to 
state  how  many  gallons  of  storm  sewage  carried  one  cubic  foot  of 
screenings. 


58  Brooklyn,  N.  Y. 

The  following  tables  are  constructed  on  the  principle  just 
stated. 

The  first  column  gives  the  time  at  which  a  bucket  holding 
8.6  cu.  ft.  of  wet  screenings  was  filled,  and  replaced  by  an  empty 
bucket. 

Operation  on  dry-weather  flow  following  the  storm,  is  in- 
cluded for  comparison. 

The  second  column  gives  the  time  in  minutes  required  to  fill 
the  bucket  with  screenings. 

The  third  column  gives  the  head  lost  by  the  flow  in  passing 
the  screen,  and  the  size  of  the  screen  apertures. 

The  fourth  column  gives  the  cubic  feet  of  flow  which  has 
passed  thru  the  screen  and  over  the  knife-edge  weir,  while  the 
bucket  was  filling. 

The  fifth  column  gives  the  gallons  of  sewage  needed  to  produce 
1  cu.  ft.  of  screenings. 

The  sixth  column  gives  the  volume  of  wet  screenings  in  parts 
per  million  of  the  sewage  out  of  which  it  was  taken  by  the 
screens,  regardless  of  its  water  content. 

Other  data  are  added  below  the  table  giving  general  results. 

TABLE  XXVIII 
R.  W .  Screen,  Typical  Operation 

North  Screen,  Apertures  5/64-inch 
June  7,   1916,  storm  began  8:00  A.  M.,  ended   11:00  P.   M. 


5/64" 
Slits 

Wet 

Time 

Quantity  Screened 

Screenings 

Bucket 

Required 

Head 

Q.  by                to  1  cu.  ft. 

to  Volume 

Filled 

to  Fill 

Lost 

Weir                 Screenings 

of  Sewage 

hour                             min.  in.  cu.  ft.                      gal.  p.p.m. 

10:00  a.  m 120  0J4  62,640  54,450  137 

11:00    60  54  31,440  27,340  274 

11:30    30  54  15,780  13,720  544 

12:00    30  54  15,840  13,770  543 

12:40  p.  m 40  y2  20,600  17,910  412 

1:30    50  y2  25,100  21,830  334 

2:10    40  54                    19,040  16,560  452 

3:30    80  V2  36,400  31,670  236 

4:45    75  54  33,750  29,380  255 

6:35     110  )/2  49,973  43,500  172 

8:00    85  y2  39,474  43,050  215 

9:50    110  y2  53,570  46,550  161 

11:00   end 70  y2  37,310  32,480  230 

Dry  weather  flow  June  8 

6:00  a.  m 420  J4  194,880  169,546  44 

8:15  p.  m 135 54 63,640 55,366  135 

Contents  of  bucket  8.60  cu.  ft. 

1.  Storm  operation,  15  hours: 

Storm-sewage  screened,  440,917  cu.  ft.  equals  3,306,900  gals. 

Screenings  removed,  13  buckets  equals  111.8  cu.  ft.  equals  33.6  cu.  ft.  per  mg. 

Minimum  flow  screened  per  cu.  ft.  of  screenings,  13,720  gals. 

Average  moisture  of  screenings  during  storm,  60   per  cent. 

Volatile  matter  in  dry  solids,  70  per  cent.;   mineral,  30  per  cent. 

2.  Dry  weather  operation,  954  hours: 

Dry-weather  flow  screened,  258,820  cu.  ft.  equals  1,940,000  gals. 
Screenings  from  dry  weather  flow,  17.2  cu.  ft.  equals  8.8  cu.  ft.  per  mg. 
Average  moisture  dry  weather  flow  screenings,  82  per  cent. 
Volatile  matter  in  dry  solids,  83  per  cent.;  mineral,  17  per  cent. 


Sewage  Treatment  Experiments 


59 


TABLE  XXIX 
R.  IV.  Screen,  Typical  Operation 

South  Screen,  Apertures  4/64-inch         July  25,  26,   1916.     See  data  below 

4/64"  Wit 

Time                                  Slits  Quantity  Screened  Screenings 

Bucket                       Required          Head  Q.  by  to  1  cu.  ft.  to  Volume 

Filled                          to  Fill              Lost  Weir  Screenings  of  Sewage 

hour                               min.                  in.  cu.  ft.  gal.  p. p.m. 
July  25  7:00  a.  m.                                            Storm  flow 

9:00    120                 0.y2  89,100  77,600  97 

1:00  p.  m 240                   yi  168,000  146,000  51 

3:30  p.  m 150                    y2  105,450  91,740  82 

4:00    30                    M  22,560  19,600  380 

4:30    30                    H  19,500  17,000  441 

6:15     end 105                    H  65,100  56,800  132 

Dry   weather  flow 

11:00  p.  m 285                    T/2  213,750  186,000  40 

July  26     4:20  a.  m 320                   y2  208,000  181,000  41 

8:30    250                    y2  160,100  139,200  54 

11:30    end 180                    y2  108,000  94,000  80 

Storm  flow 

1:00  p.  m 90                   y2  50,200  43,700  171 

1:15    p.    m 15                    3/4  8,360  7,265  1,030 

1:30    end 15                   U  8,360  7,265  1,030 

Dry  weather  flow 

9:00  p.  m 45C                    yi  312,200  271,700  28 

11:00  p.  m 120                    y2  99,600  86,600  86 

July  27     1:20  a.  m 140                   yi  116,200  101,200  74 

3:00  a.  m 100                    y2  55,800  49,550  154 

8:30  a.  m 270 y 175,500 152,600 49 

Contents  of  bucket  8.60  cu.  ft. 

1.  Storm  operation,  13 y%  hours: 

Storm-sewage  screened,  436,699  cu.   ft.   equals  4,025,000  gals. 

Screenings  removed,  9  buckets  equals  77.4  cu.  ft.  equals   19.2  cu.  ft.  per  mg. 

Minimum  flow  screened  per  cu.  ft.  of  screenings,  7,265  gals. 

Average  moisture  of  screenings  during  storm,  63  per  cent. 

Volatile  matter  in  dry  solids,  73  per  cent.;  mineral,  27  per  cent. 

2.  Dry  weather  operation,  35  y  hours: 

Dry  weather  flow  screened,   1,449,150  cu.   ft.  equals  10,868,000  gals. 
Screenings  removed,  9  buckets  equals  77.4  cu.  ft.  equals  7.5  cu.  ft.  per  mg. 
Average  moisture,  dry  weather  screenings,  80  per  cent. 
Volatile  matter  in  dry  solids,  81  per  cent.;  mineral,  19  per  cent. 

TABLE  XXX 
R.  IV.  Screen,  Typical  Operation 

North  Screen,  Apertures  3/64-inch 
August  24,  1917,  storm  began  1:20  P.  M.,  ended  8:24  P.  M. 

3/64"  Wet 

Time  Slits  Quantity  Screened  Screenings 

Bucket                       Required  Head  Q.  by  to  1  cu.  ft.  to  Volume 

Filled                          to  Fill  Lost  Weir  Screenings  of  Sewage 

hour                             min.  in.  cu.  ft.  gal.  p. p.m. 

1:35  p.  m 15  ltf.  6,264  5,450  1,372 

2:00    25  1  10,440  9,080  826 

5:10 190  H  79,344  69,000  108 

6:10    60  %  24,056  20,900  358 

6:50    40  34  16,724  14,550  514 

7:40    50  y  20,880  18,150  414 

8:25     end 45  y  18,792  16,350  457 

Aug.   25  Dry  weather  flow 

12:25  a.  m 240  y  100,224  87,300  86 

9:00    515  y  215,064  187,100  40 

8:30  p.  m 690 y 288,144 250,900 30 

Contents  of  bucket  8.60  cu.  ft. 

1.  Storm  operation,  7  hours,  5  minutes: 

Storm-sewage  screened,  176,500  cu.  ft.  equals  1,323,700  gals. 

Screenings  removed,  7  buckets  equals  60.2  cu.  ft.  equals  46.3  cu.  ft.  per  mg. 

Minimum  flow  screened  per  cu.  ft.  of  screenings,  5,450  gals. 

Average  moisture  of  screenings  during  storm,  83  per  cent. 

Volatile  matter  in  dry  solids,  87.5  per  cent.;   mineral,   12.5  per  cent. 

2.  Dry  weather  operation,  24  hours,  5  minutes: 

Dry  weather  flow  screened,  603,432  cu.  ft.  equals  4,525,700  gals. 
Screenings  removed,  3  buckets  equals  25.8  cu.  ft.  equals  5.7  cu.  ft.  per  mg. 
Average  moisture  of  dry  weather  screenings,  86.8  per  cent. 
Volatile  matter  in  dry  solids,  93.5  per  cent.-  mineral,  1.5  per  cent. 


60  Brooklyn,  N.  Y. 

TABLE  XXXI 
R.  W .  Screen,  Typical  Operation 

South  Screen,  Apertures  2/64-inch 
August  1,  1917,  storm  began  11:35  A.  M.,  ended  8:10  P.  M 

2/64"  Wet 

Time  Slits  Quantity  Screened  Screenings. 

Bucket  Required  Head  Q.  by  to  1  cu.  ft.  to  Volume 

Filled  to  Fill  Lost  Weir  Screenings  of  Sewage 


hour                             min.  in.  cu.  ft.  gal.  p.p.m. 

11:50    a.    m 17  2.  6,925  6,020  1,243 

12:50    p.    m 60  0.J4  20,904  18,200  412 

3-00                130  y2  36,270     ■  31,550  237 

4:10    70  y2  3,255  2,930  2,641 

4-45               35  H  3,255  2,930  2,641 

5:30    45  H  6,300  5,440  1,365 

8:10    end 150  y  27,900  24,300  309 

Aug.  2  Dry  weather  flow 

7:45  a.   m 705  y2  278,400  242,000  31 

11:45  a.   m 240  y  94,800  82,430  91 

11:45  p.  m 720  y2  284,500  247,500  30 

Contents  of  bucket  8.60  cu.  ft. 

1.  Storm  operation,  %l/2  hours: 

Storm-sewage  screened,   104,809  cu.   ft.   equals  786,000  gals. 

Screenings  removed,  7  buckets  equals  60.2  cu.  ft.  equals  76.6  cu.  ft.  per  mg. 

Minimum  flow  screened  per  cu.  ft.  of  screenings,  2,930  gals. 

Average  moisture  of  screenings  during  storm,  59  per  cent. 

Volatile  matter  in  dry  solids,  67  per  cent.;  mineral,  33  per  cent. 

2.  Dry  weather  operation,  27J4  hours: 

Dry  weather  flow  screened,  657,700  cu.  ft.  equals  4,932,700  gals. 
Screenings  from  dry  weather  flow,  25.8  cu.  ft.  equals  5.2  cu.  ft.  per  mg. 
Average  moisture,  dry  weather  screenings,  77  per  cent. 
Volatile  matter  in  dry  solids,  68  per  cent.;  mineral,  32  per  cent. 

Sezvage  Treatment  by  Oxidation 

So  much  space  and  time  have  been  taken  in  the  presentation 
of  sewage  data,  and  methods  of  removing  suspended  solids  from 
sewage,  that  the  subject  of  oxidation  of  sewage  must  be  elim- 
inated altogether,  or  given  very  little  space.  To  cut  it  out  en- 
tirely would  be  to  leave  the  other  data  presented  incomplete,  as 
it  is  very  desirable  to  observe  the  effect  of  tank  treatment  and 
fine-screen  treatment,  from  the  standpoint  of  the  trickling  filter 
bed.  All  that  can  be  attempted,  however,  will  be  to  give  some 
space  to  this  subject  for  that  purpose.  The  treatment  of  sewage 
by  oxidation  was  by  far  the  most  extensive  and,  perhaps,  the 
most  important  work  done  at  the  Brooklyn  experiment  station. 
The  results  of  these  studies  cannot  be  presented  in  a  limited 
space  or  at  this  time.  But  if  the  official  report  of  the  work  is  not 
published  before  the  next  annual  convention  of  this  Society,  the 
writer  will  arrange,  if  possible,  to  make  it  the  subject  of  a  pa- 
per. Under  this  head  must  also  be  included  the  treatment  of 
sewage  by  aeration  and  activation,  both  having  been  given  ex- 
tensive study  at  the  station,  but  which  on  account  of  lack  of  space 
cannot  be  included  here. 

The  term  "trickling  filter"  instead  of  "sprinkling  filter"  was 
adopted  in  conformity  with  the  recommendation  of  the  Commit- 
tee on  Nomenclature  of  the  American  Public  Health  Association. 


Sewage  Treatment  Experiments 


61 


PLAN 


TRICKLING  FILTERS  GROUP  A 


0  12  3  4  5  6 


Fig.  17. 


This  is  based  on  the  essential  features  of  the  filter  rather  than  on 
the  method  of  application  of  the  sewage,  and  therefore  seems  a 
logical  term. 

The   trickling-filter   beds    were    all    supplied    with    sewage   by 
means  of  dosing  tanks. 


62 


Brooklyn,  N.  Y. 


Each  dosing  tank  was  provided  with  a  5-inch  Miller  siphon 
which  discharged  the  dose  into  an  inverted  pyramidal  feeding 
tank,  from  the  bottom  of  which  it  was  carried,  by  a  pipe  em- 
bedded in  the  medium,  to  the  sprinkling  nozzle,  by  which  it  was 
sprayed  over  the  bed.  These  tanks  and  the  inverted  pyramidal 
feeding  tanks  into  which  they  discharge  were  constructed  of  yel- 
low pine. 


Fig.  18.     Trickling  Filters. 


The  elevation  of  the  dosing-tank  water  line  at  the  instant  of 
siphon  discharge  was  26.74  feet.  The  water-line  in  the  inverted 
pyramidal  feeding  tanks  was  controlled  by  the  amount  of  sewage 
discharged  from  the  dosing  tanks,  and  its  maximum  elevation 
with  the  largest  dose  that  could  be  discharged  was  23.50  feet, 
which  was  6  ft.  2  in.  above  the  surface  of  the  beds.  This  head 
could  be  varied  by  a  movable  bulkhead  placed  in  the  dosing  tank, 
which  increased  the  number  of  doses  per  hour. 

As  the  bulkhead  was  moved  toward  the  siphon  the  effective 
capacity  of  the  tank  was  diminished.  In  the  upper  part  of  the 
bulkhead  were  placed  circular  orifices  cut  in  thin  sheet  copper, 
each  of  which  delivered  half  a  million  gallons  per  acre  daily  on 
the  beds,  under  a  head  held  constant  by  means  of  an  adjustable 
outlet  pipe.  By  stopping  an  appropriate  number  of  these  orifices 
with  corks  the  rate  on  the  filters  was  maintained  as  desired.  These 
orifices  were  subject  to  hourly  inspection  and  did  not  show  any 
tendency  to  clog  with  Imhoff  effluent.  Their  accuracy  could  at 
any  time  be  checked  by  calibration  in  the  dosing  tank,  and  this 
was  frequently  done. 


Sewage  Treatment  Experiments 


63 


Description  of  Filters 

There  were  two  groups  of  trickling  filter  beds  which  have 
been  indicated  as  Group  A  and  Group  B.  Each  will  be  described 
separately. 

Group  A  consisted  of  the  ordinary  type  of  trickling  filters. 
There  were  four  beds.  The  foundations  were  carried  on  piles, 
and  the  bottoms  of  the  beds  were  6  feet  above  the  level  of  the 
marshland  over  which  they  were  built.  High  water  at  unusually 
high  tides  washed  underneath,  at  times  covering  the  marsh  to  a 
depth  of  6  or  8  inches.  Each  bed  was  square  and  had  an  effective 
area  of  .005  of  an  acre. 

Partitions,  4  inches  thick,  were  carried  from  the  floor  to  the 
surface  of  the  medium  between  adjacent  beds. 

The  outer  walls  of  the  beds  were  formed  by  means  of  rein- 
forced concrete  piers,  carrying  a  slab  coping  of  reinforced  con- 
crete at  the  tip.  The  piers  were  cast  with  slots  for  receiving 
3-inch  yellow-pine  slabs  or  shutters,  which  were  set  at  an  angle 
of  45  degrees,  sloping  inward,  spaced  2  feet  apart,  against  which 
the  medium  rested,  affording  a  maximum  admission  for  air  and 
preventing  the  escape  of  sewage. 

The  floor  was  formed  of  concrete  with  a  slight  slope  to  the 
outlet  of  the  underdrains.  Half-tile,  6  inches  in  diameter,  were 
laid  with  the  convex  sides  up  on  the  concrete  floor  to  afford 
drainage ;  the  effluent  flowing  to  gutters  or  troughs  placed  around 
the  bottom  of  the  beds,  outside  of  the  wall  piers.  Each  bed  had 
its  individual  gutter,  which  discharged  by  means  of  a  pipe  or 
flume  to  its  individual  secondary  settling  tank. 


Fig.  19.     Trickling   Filter  Bed,  after  Two  Years'  Service. 


64 


Brooklyn,  X.  Y. 


■   ■      .      H-f  . 

L    jfe  38                                  1 

6 

a 

CD 

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Fig.  20.     Trickling  Filter   Bed  after  Two   Years'   Service. 

The  medium  was  10  feet  in  depth  over  the  top  of  the  under- 
drains,  and  of  very  carefully  selected  broken  trap-rock,  many 
runs  thru  screens  having  been  necessary  to  obtain  the  result  re- 
quired. 

In  order  to  prevent  the  effects  of  wind  on  the  sewage  dis- 
tribution, a  shield  was  provided,  consisting  of  a  board  fence  at 
the  surface  carried  between  the  beds  and  around  them. 

In  order  that  the  effect  of  the  depth  of  filter  medium  under 
similar  conditions  of  operation  might  be  obtained,  test  trays  were 
placed  in  the  filter  beds  at  different  depths.  The  trays  were  so 
placed  that  samples  could  be  taken  at  depths  from  the  surface  of 
4  feet,  6  feet,  7  feet  3  inches,  and  8  feet  6  inches.  Samples 
taken  from  the  bottom  of  the  bed  give  the  result  of  10  feet 
depth. 

For  the  distribution  of  sewage  to  the  surface  of  the  filter,  sev- 
eral types  of  nozzles  were  tried,  but  for  the  majority  of  the  ex- 
periments, which  were  at  high  rates,  the  Taylor  square  spray  and 
the  Worcester  nozzles  were  employed. 


Sice  of  Medium  in  Filter  Beds,  Group  A 

Bed  No.  1,  stone  passing  ring  \y2  inches  in  diameter,  retained 
by  ^4-inch  ring. 

Bed  No.  2,  stone  passing  ring  2  inches  in  diameter,  retained 
by  1-inch  ring. 

Bed  No.  3,  stone  passing  ring  2l/2  inches  in  diameter,  retained 
by  \l/4 -inch  ring. 

Bed  No.  4,  stone  passing  ring  2l/2  inches  in  diameter,  retained 
by  1  ^4-inch  ring. 


Sewage  Treatment  Experiments 


65 


Fig.  21.     Trickling   Filter  Bed  after  Two  Years'  Service. 


Fig.  22.     Trickling  Filter   Bed  after  Two   Years'   Service. 


Group  B  was  a  tank  12  feet  in  diameter  and  16  feet  high  in 
which  were  placed  two  beds.  A  partition  wall  divided  the  tank 
into  two  equal  parts  and  each  side  was  filled  with  stone  filtering 
medium  to  the  same  depths.  The  bottom  of  each  side  was  under- 
drained  with  6-inch  half-pipe  tile,  on  a  concrete  bed,  and  closed 
from  external  air  by  the  tank  walls.  Each  side  was  kept  entirely 
from  the  other,  and  drained  independently  to  a  secondary  tank. 
Each  side  was  provided  with  a  grid  for  supplying  compressed 
air,  placed  within  the  medium  near  the  bottom,  formed  of  j^-inch 


66 


Brooklyn,  N.  Y. 


iron  pipe,  perforated   every  6  inches   with    3^-inch  holes,   thru 
which  compressed  air  could  be  supplied. 

In  operation  the  sewage  was  sprayed  upon  the  surface  by 
a  single  nozzle  placed  at  the  center  of  the  two  beds,  over  the 
dividing  wall  between  them.  Two  designs  of  Taylor  circular- 
spray  nozzles,  and  the  Worcester  nozzle,  were  tried  during  the 
course  of  the  experiments.  Both  beds  could  be  operated  as  or- 
dinary trickling  filter  beds,  in  which  case  air  entered  the  bed 
from  the  surface  only.  Compressed  air  could  be  supplied  to  both 
beds  at  the  same  time,  or  one  side  could  be  operating  as  an  ordi- 
nary trickling  filter,  while  the  other  was  operated  as  a  trickling 
filter  with  compressed  air  added  in  the  bed,  in  order  that  the 
effluents  might  be  compared,  and  the  effect  of  the  added  air  ob- 
served. The  filtering  medium  was  of  the  best  selected  broken 
trap-rock.  The  depth  of  the  medium  was  10  feet,  and  might  be 
increased  up  to  16  feet,  if  desired. 


Fig.  23.     Surface  of  Trickling  Filter  3,   after  Six   Months'   Service  on 
Screened    Sewage. 

Size  of  Medium  in  Filter  Beds,  Group  B 

Bed  No.  5,  stone  passing  ring  2l/2  inches  in  diameter,  retained 
by  1^4 -inch  ring. 

Bed  No.  6,  stone  passing  ring  2y2  inches  in  diameter,  retained 
by  l*4-inch  ring. 

Size  and  Character  of  Stone  for  Trickling  Filters 

Every  effort  was  made  to  obtain  an  accurate  knowledge  of  the 
stone  which  was  put  into  the  filters.  To  this  end,  very  frequent 
samples  were  taken  during  the  delivery  of  the  material  at  the 


Sewage  Treatment  Experiments'  67 

plant,  and  all  stone  which  did  not  conform  very  closely  to  the 
provisions  of  the  specifications  were  rejected  and  the  contractor 
was  required  to  re-screen  it. 

The  specifications  for  filter  No.  4,  calling  for  stone  from  2-inch 
to  2^-inch  size,  were  found  too  severe,  when  the  effort  was 
made  to  screen  the  stone  to  the  specified  size ;  so,  after  many  un- 
successful attempts,  the  contractor  was  allowed  to  furnish  stone 
which  would  pass  a  2^2-inch  ring  and  be  retained  upon  l^-inch 
ring. 

The  following  table  shows  the  percentage  by  weight  of  the 
stone  in  each  filter  which  did  not  lie  within  the  specified  limits : 

Filter  No.  1 4.7% 

Filter  No.  2 6.3% 

Filter  No.  3,  5,  6 2.3% 

Filter  No.  4 3.7% 

The  data  obtained  by  analyses  of  samples  were  as  f^Uows : 
(1)  median  size,  (2)  uniformity  number,  (3)  superficial  area, 
and  (4)  per  cent,  of  voids. 

The  "median  size"  is  a  number  somewhat  analogous  to  the 
"effective  size"  of  filter  sand,  but  is  defined  as  follows :  it  is  the 
diameter  of  a  ring  of  such  size  that  fifty  per  cent,  (by  number) 
of  the  stones  will  be  retained  upon  it  and  fifty  per  cent,  pass  it. 
It  is  generally  not  much  different  from  the  mean  of  the  diameters 
of  the  rings  defining  the  sample  in  the  specifications.  The  "uni- 
formity number"  is  analogous  to  the  "uniformity  coefficient" 
used  in  water  filter  sand,  and  is  the  ratio  of  the  diameter  of  the 
ring  thru  which  75  per  cent,  (by  number)  of  the  stones  will  pass 
to  that  of  the  ring  thru  which  25  per  cent,  will  pass.  It  ap- 
proaches 1.00  as  the  material  approaches  perfect  uniformity,  and 
increases  with  the  non-uniformity  of  the  sample.  The  per  cent, 
of  voids  was  determined  by  weighing  the  water  necessary  to  fill 
the  void  spaces  in  an  amount  of  stone  whose  gross  volume  was 
known. 

TABLE  XXXII 

Properties  of  Broken  Trap  Rock  For  Use  in  Filters 

Filter  1  Filter  2      Filters  3,  5,  6     Filter  4 

Specified   size   (inches) V/2-H  2-1  2J4-1J4  lYi-Wi 

Median    size    (inches) .94                  1.32  1.83  2.14 

Uniformity    number 1.50                  1.56  1.42  1.20 

Superficial  area  per  ton   (sq.   ft.) :....  1025  711  568  465 

Superficial  area  per  cu.  yd.   (sq.  ft.) 1343  945  741  579 

Per  cent,   void   space _ 46.8  45.0  46.5  47.0 

On  examining  the  above  table,  it  will  be  noted  that  the  super- 
ficial area  is  roughly  proportioned  to  the  reciprocal  of  the  median 
size.  This  would,  of  course,  be  strictly  true,  if  the  material 
consisted  of  uniform  spheres. 

The  most  uniform  sample  (filter  No.  4)  has  the  highest  per- 
centage of  voids,  and  the  least  uniform  (filter  No.  2)  has  the 
lowest  percentage,  which  is  also  what  should  be  expected. 


68 


Brooklyn,  N.  Y. 


Determination  of  Stone  Surfaces 

Since  the  degree  of  purification  obtained  in  a  trickling  filter  is 
dependent,  among  other  factors,  upon  the  number  of  bacteria 
which  can  act  upon  the  sewage,  and  the  number  upon  the  amount 
of  surface  upon  which  they  can  multiply,  a  knowledge  of  the 
superficial  area  of  the  stone,  per  ton  or  per  cubic  yard  is  desir- 
able. Hitherto,  so  far  as  the  writer's  knowledge  goes,  no  one 
has  published  any  data  upon  this  subject,  and  an  attempt  was 
made  here  to  arrive  at  knowledge  of  it,  and  the  laws  of  its 
variation. 

The  method  employed  to  obtain  this  information  was  as  fol- 
lows :  Samples  of  broken  granite  and  of  broken  trap  were  ob- 
tained and  screened  thru  sieves  having  rings  of  the  following 
diameters:  2y2" ,  \]/A" ,  1",  %",  J/2".  About  100  stones  of  each 
size  were  kept  for  area  and  weight  determination.  The  area  was 
determined  by  tracing  with  a  sharp  pencil  the  shapes  of  the  faces 
of  the  stones  on  a  sheet  of  paper.  As  the  faces  of  the  stones  are 
almost  always  nearly  plane,  this  presented  no  serious  difficulty, 
aside  from  extreme  laboriousness.  The  areas  were  then  deter- 
mined by  planimeter. 

To  enable  the  information  to  be  applied  to  the  various  sizes 
of  stone,  as  put  into  the  filters,  the  ratio  of  the  mean  area  to  the 
area  of  a  sphere,  of  the  same  material  of  equal  weight  (or  equiv- 
alent sphere),  seemed  the  easiest  to  use.  The  following  table 
Sfives  the  results  of  the  studv : 


TABLE  XXXIII 
Analyses  of  Stone  for  Trickling  Filter  Beds 

Weight  Surface    

Ratio  of 
Pieces  mean  surf. 

Sizes  in  Total  Average  Total  Average      to  surf,  of 

Inches  sample  grams  grams  sq.  ft.  sq.  ft.     equiv.  sphere 

H-V2 

Granite  100      265       2.65       .561      .0056      1.08 

Trap  100      415       4.15       .881      .0088      1.12 

1-H 

Granite    102  630  6.18  1.049  .0103  1.12 

Trap     100  829  8.29  1.423  .0142  1.30 

154-1  , 

Granite     98  1320  13.46  f         2.115  .0216  1.40 

2^-i  yA 

Trap      100  1855  18.55  2.428  .0243  1.29 

Granite     108  7289  67.50  6.049  .0560  1.24 

Trap      100  6330  63.30  5.015  .0502  1.18 

The  filter  beds  were  made  of  trap  rock  exclusively.  In  the 
above  table,  results  of  analyses  from  granite  samples  are  included 
for  comparison. 

Observation  of  the  samples  shows  that  among  the  pieces  there 
are  a  greater  or  less  number  of  stones  which  are  ''scales"  or 
"plates",  whose  thickness  is  small  compared  with  the  other  di- 


Sewage  Treatment  Experiments  69 

mensions.  The  relatively  high  values  of  the  above  ratio- in  the 
sizes  from  1/4"  to  1*4"  indicate  a  rather  high  proportion  of  these 
thin  stones  in  the  above  mentioned  sizes. 

As  the  stone  for  the  filters  was  delivered,  samples  were  taken 
and  screened  by  sieves  of  plate  containing  circular  holes  having 
diameter  varying  by  %" .  The  total  weight  of  a  size  of  separation 
divided  by  the  number  of  stones,  gives  the  mean  weight,  from 
which  the  area  of  the  equivalent  sphere  may  be  found  from  the 
following  formula : 


A  =  J363-",16W'x  .0010765 


\ 

in  which  A  is  area  of  equivalent  sphere  in  square  feet;  W,  the 
mean  weight  of  a  single  stone  is  grams ;  g,  the  specific  gravity  of 
the  rock ;  and  .0010764  the  constant  for  reducing  sq.  cm.  to  sq.  ft. 
A  suitable  multiplier  selected  from  the  following  table  enabled 
the  true  mean  superficial  area  corresponding  to  the  mean  weight 
to  be  calculated. 

TABLE  XXXIV 

Ratio  of  mean  area  to 

Mean  weight  area  of  equiv.  sphere 

Grams 

4 1.12 

S 1.15 

6 1.20 

7 : 1.2S 

8 1.29 

9  to  20 1.29 

2S 1.27 

30 1.26 

35 / 1.25 

40 1.24 

45 1.22 

50 1.21 

55 1.20 

60 1.19 

80 1.18 

100 1.17 

120 1.15 

140 1.14 

160 1.13 

180 1.12 

200 1.10 

220 1.09 

This  table  of  multipliers  was  interpolated  from  the  data  in 
the  first  table,  and  from  an  area  determination  performed  subse- 
quent to  the  first  studies.  The  first  table  carried  the  knowledge 
of  the  ratio  only  to  a  mean  weight  of  60  grams,  and  the  subse- 
quent determination  enabled  it  to  be  known  when  the  mean  weight 
was  225  grams. 

Having  the  mean  area  of  a  single  stone  and  the  number,  of 
stones  of  that  separation  size  in  the  sample,  it  is  simple  to  com- 
pute the  total  area  per  ton,  and  after  weighing  a  known  volume 
of  the  sample,  to  compute  the  total  area  per  cubic  yard,  as  put 
into  the  filter. 


70 


Brooklyn,  N.  Y. 


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Sewage  Treatment  Experiments 


71 


The  following  numerical  example  will  illustrate.  The  screen 
analyses  of  the  stone  put  into  filters  No.  3,  5  and  6  gave  the  fol- 
low data : 


Passing 


Size 


Retained  by 


Total  Weight 
lbs. 


No.  Stones 


2/2 

2 

154 

I/2 

1 


2 

m 

Hi 
1 


118.5 

67.5 

23.2 

16.0 

5.0 

.27 


306 
247 
145 
157 

85 
7 


Beginning  with  the  2y^ 
118.5x454 


-2  size,  the  mean  weight  of  a  single 


stone  is 


306 


=  176  trains.     From  the  formula  for  sur- 


face-equivalent sphere,  we  find  this  weight  corresponds  to  .0843 
sq.  ft.  From  the  table  of  ratios,  we  find  the  proper  multiplier 
for  size  176  grams  to  be  1.12;  hence  the  true  mean  area  of  a 
single  stone  is  .0943  sq.  ft.  This  multiplied  by  306  gives  28.9 
sq.  ft.  for  the  area  of  the  stones  of  the  sample  lying  between  21/z'r 
and  2" .  Proceeding  in  the  same  way,  we  find  the  total  area  of 
the  size  2 — 1^4  to  be  18.8  sq.  ft.;  of  the  size  l-}4 — 1^  to  be 
8.0  sq.  ft.;  of  the  size  \l/2  to  1%  to  be  6.6  sq.  ft.;  of  the  size 
1^4  to  1  to  be  2.9  sq.  ft.;  and  of  the  size  1 — ^4  to  be  2  sq.  ft., 
making  a  total  area  for  the  sample  (which  weighed  230.47  lbs.) 
of  65.4  sq.  ft.,  or  568  sq.  ft.  per  ton.  It  was  found  that  the  stone, 
as  placed  in  the  filter,  weighed  1.305  tons  per  cubic  yard,  so  that 
the  area  amounted  to  741  sq.  ft.  per  cubic  yard. 

Operation  of  Trickling  Filters 
Arrangement  of   the  Beds   and   Final   Settling  Tanks : 

Filter  No.  1  Discharges  thru  Tank  No.  6 
Filter  No.  2  Discharges  thru  Tank  No.  10 
Filter  No.  3  Discharges  thru  Tank  No.  9 
Filter  No.  4  Discharges  thru  Tank  No.  7 
Filter  No.  5  Discharges  thru  Tank  No.  5 
Filter  No.  6  Discharges  thru  Tank  No.  8 
Area  of  beds,  Nos.  1  to  4,  inclusive,  .005  acre 
Area  of  beds,  Nos.  5  to  6,  inclusive,  .00127  acre 

During  the  course  of  the  experiments  the  sewage  distributed 
on  the  trickling  filters  consisted  of  (1)  Imhoff  tank  effluent; 
(2)  crude  sewage  which  had  been  subjected  to  fine  screening. 
The  former  was  experimented  with  from  November  1,  1913, 
when  the  filters  were  first  put  into  operation,  to  December  15, 
1915.  During  the  latter  part  of  December,  1915,  and  during  the 
winter  and  spring  of  1916,  the  filters  were  shut  down,  pending 
the  completion  of  the  Riensch-Wurl  screen  plant.    Crude  sewage, 


72 


Brooklyn,  N.  Y. 


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Sewage  Treatment  Experiments 


73 


which  had  passed  these  screens,  was  applied  to  the  beds  of  Group 
A,  beginning  June  1,  1916,  and  this  method  of  operation  was  con- 
tinued to  the  middle  of  January,  1917,  when,  owing  to  the  failure 
of  the  pump,  it  was  necessary  to  shut  down  until  March  1,  when 
operation  began,  first  on  Imhoff  tank  effluent  for  a  month,  then 
on  screen  effluent  for  the  rest  of  the  year. 

The  results  obtained  from  the  application  of  Imhoff  tank 
effluent,  and  from  that  of  screened  crude  sewage,  are  given  in 
sufficient  detail  to  enable  a  comparison.     See  tables  27,  35,  36. 

Experiments   With  Imhoff1   Tank  Effluent  on   Trickling  Filters 
Analytical  observations  date  from  April  1,  1914.     Previous  to 
this  the  niters  were  undergoing  a  period  of  tuning  up,  and  their 
mechanical  operation  was  being  studied. 

When  first  put  in  service,  all  the  niters  were  operated  at  the 
rate  of  2,000,000  gallons  per  acre  daily,  which  was  continued 
during  the  time  of  preliminary  trial.  This  rate  was  maintained 
on  the  niters  of  Group  A  from  April  1,  1914,  to  October  1,  1914. 
The  filters  of  Group  B  were  operated  at  varying  rates  from  two 
to  five  million  gallons  per  acre  daily. 

On  October  1,  1914,  filters  Nos.  2  and  3  were  set  operating  at 
4,000,000  gallons  per  acre  daily ;  filters  1  and  4  remaining  at 
2,000,000.  These  rates  were  carried  until  the  end  of  1917.  Fil- 
ters 5  and  6  were  operated  from  the  spring  of  1915  at  the  rate 
of  4,000,000  gallons  per  acre  for  the  remainder  of  the  experi- 
ments. 

FINAL  SETTLING  TANKS  (humus) 
PLAN 


?-A 


rtrr 

SECTION     A-A 


Fiff.  24. 


74 


Brooklyn,  N.  Y. 


Final  settling  tanks  proved  to  be  essential  in  the  treatment 
of  the  effluent  from  the  trickling  filters.  The  effluent  carried 
very  considerable  quantities  of  floculent,  but  very  readily  set- 
tleable  materials  in  suspension,  also  the  remains  of  animal  life 
from  the  beds,  in  the  form  of  dead  worms  in  masses,  as  well  as 
living  ones.  The  amount  of  suspensa  carried  by  the  filter  effluent 
were  at  times  surprising,  when  for  any  reason  an  unloading  of 
films  derived  from  the  medium  appeared.  The  sludge  settled  out 
in  the  settling  tanks  at  times  made  all  the  difference  between  a 
putrescible  and  a  non-putrescible  effluent  from  the  plant.  This 
sludge  was  highly  putrescible  and  very  difficult  to  dry.  It  should 
be  returned  as  a  rule  to  primary  tanks  for  full  treatment. 

The  quantity  of  settlings  removed  by  these  tanks  seemed  to 
be  about  the  same  whether  the  sewage  had  passed  an  Imhoff 
tank  or  a  fine  screen  before  filtration,  and  to  depend  mostly  if 
not  altogether  on  filter  conditions  and  phenomena. 


ABC 

Fig.   25.     Trickling   Filter   Effluent. 

The  bottles  are  about  16  inches  high  and  are  stood  against  a  sheet 
of  white  cardboard  supporting  a  line  of  black. 

A — Crude  Sewage.     Settleable  solids  about  200  P.  P.  M. 

B — Imhoff  Tank  Effluent.     80%  removal  of  settleable  solids. 

C — Unsettled  Effluent  from  7  ft.  3  in.  depth  of  filter.  Relation 
stability  99%  to  100%  average. 


The  settling  tank  following  filter  No. 3  removed  in  1  hour's 
retention,  from  October,  1914,  to  September,  1915,  62  per  cent, 
of  the  solids  in  suspension  in  the  filter  effluent.  The  settling 
tank  following  filter  No.  1,  removed  in  2  hours'  retention  90 
per  cent  of  the  solids,  and  the  settling  tank  following  filter  No. 
5  removed  in  3  hours'  retention  70  per  cent,  of  the  effluent  sus- 
pensa. With  these  tanks  in  operation  bacterial  removal  approx- 
imated 99  per  cent,  and  the  plant  effluent  was  100  per  cent,  stable. 


Sewage  Treatment  Experiments  75 

Concluding  Remarks 

In  concluding  this  account  of  his  work,  the  writer  feels  as 
tho  after  much  labor  he  had  accomplished  but  little.  At  most, 
he  has  obtained  some  fragmentary  information  as  to  what  tanks, 
screens  and  filters  will,  and  especially  what  they  will  not  do;  but 
these  are,  after  all,  rather  negative  data.  The  knowledge  derived 
from  one  plant  can  only  be  of  partial  and  unbalanced  character. 
Observations  from  many  plants  obtained  in  accordance  with  a 
definite  system  of  investigation  employed  in  each  instance,  or 
universally,  would  be  of  real  value  and  would  advance  science  to 
a  higher  plane.  Unfortunately,  there  is  not  that  uniformity  now 
in  practice  which  will  give  such  a  result.  Much  has  been  done  to 
advance  this  object,  however,  especially  by  such  institutions  as 
the  Massachusetts  Institute  of  Technology,  the  American  Public 
Health  Association,  and  other  agencies  of  progress  that  might 
be  mentioned.  The  work  of  some  of  our  consulting  engineers 
has  also  been  very  fruitful  along  these  lines.  The  books  of 
Messrs.  Metcalf  and  Eddy,  and  of  George  W.  Fuller  have  accom- 
plished much;  but  more  is  needed,  especially  in  the  way  of  co- 
operation among  engineers  and  sanitarians,  and  more  quiet,  studi- 
ous discussion  among  these  men. 

Looking  at  these  problems  from  the  standpoint  of  a  mere  muni- 
cipal engineer,  the  writer  has  never  felt  and  does  not  now  feel 
satisfied  with  the  present  status  of  sewage  disposal, — that  neces- 
sary evil  that  must  be  compromised  with — under  the  present  state 
of  our  knowledge.  As  to  tanks  of  all  kinds,  they  are  abominable 
to  many,  and  so  also  are  screens  of  all  kinds.  "It  cannot  be  that 
these  are  the  only  possible  methods  of  treating  sewage" !  Nor 
is  the  trickling  filter  a  friend  to  all.  As  the  pot  calls  the  kettle 
black,  so  the  ancient  House  of  Tanks  and  the  aspiring  House  of 
Screens  belabor  each  other  with  bad  names,  out  with  the  lot! 
"A  plague  on  both  your  houses !'  Give  us  something  new  and 
without  fault.  Possibly  the  future  will  produce  such  a  method 
of  sewage  treatment  that  this  age  will  seem  the  age  of  barbarism, 
and  the  following  ideal  of  such  a  plant  shall  close  this  paper: 
It  shall  be  a  simple  electrically  operated  machine,  placed  in  a 
fine  large  hall,  with  potted  palms  at  convenient  places  for  decora- 
tion. The  sewage  will  enter  from  below  and  will  rise  up  thru 
the  plant,  being  discharged,  after  treatment,  a  pure  pellucid 
stream  of  water,  cascading  down  to  the  nearby  city  reservoir. 
Meanwhile,  from  one  side  of  the  plant,  in  neatly  done  up  bundles, 
will  come  forth  automatically  compressed  fertilizer,  containing 
nitrogen  units  enough  to  pay  all  expenses. 


CONTENTS 


Introduction    3 

A  Brief  Review  of  the  Work 5 

Plan  of  the  Plant 6 

Mechanical   Equipment   8 

Laboratory  Control  and  Sampling 9 

Definitions  of  Terms 10 

Local  Conditions  12 

Measurement   of   Sewage : 13 

Calibrated   Orifices   - 14 

Measurement  of  Compressed  Air 15 

Distribution  Control 17 

Obtaining  Sewage  for  the  Tests 18 

Population  and  Daily  Flow  of  Sewage 19 

Per  Capita  Flow  for  Each  Hour  and  Day, 20 

Storm  Water  Flow,  Character  of 21 

Sewage,    Character   of 22 

Imhoff  Tanks  and  Settling  Tanks 25 

Type  of  Slot  Used  in  Imhoff  Tanks 26 

Slope  Inclination,  Imhoff  Tanks 26 

Period   of    Ripening 30 

Foaming  and  Odors 30 

Neglect  of  Sewerage  System  Cause  of  Odors 32 

Dimensions  of  Tanks 33 

Theoretic  and  Observed  Retention 34 

Effects  of  Baffles  in  Tanks 35 

Character  of  Floating  Scum 36 

Bacterial  Content  of  Sewage  and  Effluent 36 

Settling  Tank  and  Separate  Digestion  Tank 38 

Capacity  of  Digestion   Chamber   Per   Capita 43-45 

Sludge  Drying  Bed,  Area  Per  Capita 44 

Sewage  and  Effluent  Data 46 

The  Riensch-Wurl  Screens 49 

Requirements  of  Contract  for  Screens : 50 

Results  of  Operation  of  Screens 51 

Screened  Sewage  on  Trickling  Filter  Beds 54 

Cleaning  Screens 56 

Removal  of  Screenings 56 

Analyses  of  Screenings 56 

Weight   of   Screenings 56 

Typical  Operation  of  Screens,  Storm,  and  Dry  Weather 57 

Sewage  Treatment  by  Oxidation 60 

The  Trickling  Filter  Beds 61 

Size  and  Character  of  Stone  in -Filters 66 

Determination  of  Area  of  Stone  Surfaces 68 

Summary  of  Trickling  Filter  Results 70-72 

Final   Settling  Tanks , 74 

Concluding    Remarks 75 


TABLES 


Table  Page 

I— Population  of  26th  Ward,  Brooklyn,  N.  Y 19 

II — Daily  and  Hourly  Flow  of  Sewage 19 

III — Per  Capita  Flow,  for  Every  Hour,  Day,  and  Week 20 

IV — Storm  Water  Flow,  Character  of , 21 

V — Sewage,    General    Characteristics - 22 

VI — Sewage,  Oxygen  Relations 22 

VII — Sewage,   Nitrogen   Content 23 

VIII — Sewage,  Cycle  of  Changes  in  Strength 23 

IX — Sewage,  Volume  of  Deposit  and  Time  of  Settling 25 

X — Tank  Data,  Imhoff  Tank  Dimensions 33 

XI — Tank  Data,  Settling,  Digestion  and  Humus 33 

XII — Tank  Data,  Theoretic  and  Observed  Retention 35 

XIII — -Floating  Scum,  Imhoff  Tanks 36 

XIV — Effect  of  Tank  on  Bacterial  Count 37 

XV — Effect  of  Tank  on  Bacterial  Count  Under  Storm  Conditions 38 

XVI— Crude   Sewage   Supplied    Plant   1914-15 46 

XVII— Effluent,  Imhoff  Tank  1 46 

XVIII— Effluent,  Imhoff  Tank  2 47 

XIX— Effluent,  Imhoff  Tank  3 47 

XX— Effluent  Plain  Settling  Tank  3 48 

XXI — Percentages  of  Removal  Compared 48 

XXII — Sludge   Drying  Data,    Summary   of 48 

XXIII— Riensch-Wurl  Screens,   Official  Test 51 

XXIV — Screens  and  Tanks  Compared 52 

XXV — Screens    and   Tanks    Compared.      Tank   Operating    on    Strong 

and  Screen  on  Weak  Sewage 53 

XXVI — Screens  and  Tanks  Compared.     Tank  and   Screen   Operating 

on  Same  Sewage  54 

XXVII — Screen  Effluent  on  Trickling  Filter  Beds 54 

XXVIII — Typical  Operation  of  Screen  5/64  Inch  Apertures 58 

XXIX- — Typical  Operation  of  Screen  4/64  Inch  Apertures 59 

XXX — Typical  Operation  of  Screen  3/64  Inch  Apertures 59 

XXXI — Typical  Operation  of  Screen  2/64  Inch  Apertures 60 

XXXII — Trickling  Filters,  Properties  of  Trap  Rock  Used 67 

XXXIII— Analysis  of  Stone  Particles   for  Filters 68 

XXXIV — Mean  Weight  of  Particles  and  Ratio  to  Spheres 69 

XXXV — Trickling  Filters — Summary  of  Results  April,  May,  June 70 

XXXVI — Trickling  Filters — Summary  of  Results  July,  Aug.,  Sept 72 


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